The Game Design Forum

The Cover Theme Part 1

Introduction and Structure

The cover theme in Half-Life is made up of seven segments, each composed of several set pieces. Each segment shows the player variations on a single idea. The first segment teaches the player how to use cover and features mostly obvious object-type cover (like crates, boxes, etc.) from which the player does not have to move. The second segment is similarly structured, although the cover is architectural rather than of the object type. In the third segment, the player is required to move between multiple pieces of cover. Whereas the first two segments generally allow the player to find a convenient piece of cover and stay behind it for a while, the third segment requires the player to shift between multiple pieces of cover in order to find the best angle from which to fire on the enemies. The fourth segment causes the player to shift between different pieces of cover by surprising the player with a variety of unusual enemy placements and enemy behaviors. The fifth segment goes beyond this, forcing the player to move from cover to cover constantly because that cover will often be destroyed or its protection negated by concentrated, high-explosive munitions fire. The sixth segment goes the furthest of all, and puts the player in situations where Freeman is surrounded on all sides, meaning that constant re-adjustments are necessary to take advantage of any cover at all. (There is also one final segment in Xen, but it consists of assorted ideas that are never developed.)

For convenience, here is a list of each segment and how it aims to improve/test the player’s cover discipline.

  1. Identifying basic cover (mostly objects), and staying in it.
  2. Identifying subtler architectural cover
  3. Moving between multiple pieces of cover to get the best angle on a set piece
  4. Reassessing cover quickly after a deceptive initial engagement
  5. Fleeing from temporary cover
  6. Fighting in cover while surrounded
  7. [Fragmentary] Moving cover, and cover which is itself dangerous

It's easy to see how the overall trend is towards greater danger and more movement between pieces of cover, but the use of cover never becomes obsolete. In each segment, we'll see how different features are introduced and fall out of use as the game goes on. The biggest two features are the cover descriptor and the practice of cover discipline. The cover descriptor is an introductory feature that tells players how to "read" the cover in a set piece. Usually the cover descriptor takes the form of a conspicuously placed object like a large box or laser beam. From the placement of this object the player can figure out how the entire set piece operates. Here’s an example of a cover-describing object, and then beside it I’ve put an example of how this would be handled in a current game.

Of course, there are not floating icons like this in in Half-Life. In this set piece, a scientist and Headcrab are blown to bits, demonstrating the necessity of this piece of cover. We’ll examine a variety of cover descriptors in detail in the first two segments, but descriptors become more scarce and more abstract in later set pieces. Cover descriptors never disappear entirely, however, and we’ll see a few examples of their role in later set pieces too.

Cover discipline, on the other hand, is a skill that the cover descriptor helps to teach early in the game. Cover discipline is the ingrained practice of staying in cover despite the appearance of pressuring enemies or events, finding the best angle to fire upon enemies from and eliminating the closest and most dangerous enemies first. Cover discipline also means knowing when to back into the cover to avoid damage. Eventually, cover discipline comes to mean knowing when to abandon cover and move to the next piece of it because a grenade has landed or several enemies are converging on Freeman's position. At its most complex, the kind of cover discipline needed in segment 6 means always standing in the best location to take advantage of a little bit of cover while fight on multiple sides. Interestingly, there are a couple of set pieces in Xen which take cover discipline to new and strange places, but they don't do it in any regular way, and so don't constitute a thematically linked segment. Nevertheless, we'll see what cover discipline means when the cover is moving, or when destroying the cover means releasing additional enemies.

Segment 1: Basic Cover

This segment is very simple, and is focused on merely teaching the player how to use cover. In 1998, cover of the kind that Half-Life was offering would have been completely novel, and its use would have seemed alien to anyone who had grown up on the arena shooter. Thus, we see many instances of the game telegraphing the direction and quality of fire before the player even has to enter the set piece. Cover in this segment is almost always in the form of large, obvious boxes. Most of the enemies in this set piece are static turrets that, while highly damaging, do not move and cannot charge Freeman’s position to negate his cover. In the middle of this segment, the designers use teleporting enemies to try and force the player to break cover discipline, but none of the teleports are terribly threatening on their own, usually consisting of one enemy. The goal of these teleports is to teach the player not to panic, which is one of the first and most important lessons a game can teach a player.

SET PIECE 4-2: Cover!

The cover theme begins with a simple exercise in obvious, one-directional fire and one type of cover. The source of the fire is an automated turret that puts out a lot of damage and isn't immediately visible upon entering the set piece.

The designers do a great job here of alerting the player to the danger here with two elements that control the player's movement. The stopping mechanism is the vertical drop out of the crawlspace which will probably cause the player to pause long enough to see the scripted sequence (the scientist and Headcrab being messily shot to pieces) which is the cover descriptor. The only really difficult thing about this set piece is that it's a little hard to see the layout of the cover from the vent aperture. The player has to either run clear across the set piece into the safe zone on the other side, or drop and duck immediately behind the right-hand cover. Either move is a kind of "leap of faith" that first-time players might be reluctant to take. Taking the leap will teach the player something very important about the use of cover: that enemies don't start shooting immediately. This is one of the facts that make the use of cover possible. Without that important delay, jumping out of cover to move or fire would be impossible, and players would be paralyzed.

The other apparently difficult thing about this set piece isn't a difficulty at all, but rather a constraint that helps to teach the player about using cover. The turret is essentially impossible to kill at a distance, and it's pretty obvious that trying to fight it is a bad idea. The only real option is to use the cover to hide from it. This is ideal, because it forces new players to focus on their hiding rather than their shooting. The combination of hiding and shooting will come later. As a reward for players who figure out that some of the cover objects are moveable, there's a panel allowing the player to disable the turret beneath its platform, and a health station above. This is an ideal example of "bonus goals" because it's totally in keeping with the theme of hiding rather than fighting.

SET PIECE 4-3: Office Combat

Although the Vortigaunts in this set piece do move and will charge at Freeman, they still don't require the player to do that much moving. The many offices in this section of the level serve perfectly as accessory spaces. An accessory space is an empty room with a narrow aperture that gives the player a perfect location to engage enemy units. (The best one here is marked “AC” on the map below.) The rooms are so handy and so obvious that they serve as their own cover descriptors. The player simply draws the aggro of the Vortigaunts and then parks Freeman in any of the several spaces indicated below.

The Vortigaunts will line up perfectly in these narrow apertures to be shot. The player doesn't have to do that much backtracking because there are so many good spaces designed right into the architecture. While the cover here isn't made up of the same boxes as elsewhere in the segment, it's nevertheless obvious that these rooms and narrow apertures can make it easy for the player to stop and fire on groups of disadvantaged enemies.

SET PIECE 5-1: A Sleeping Turret

Here we have our first look at the puzzle aspect of cover and the first good example of a geographic cover descriptor (rather than a script which performs a revealing action). The turret in this set piece starts in a dormant state and is alerted by Freeman crossing the laser tripwires. The turret will immediately spring to life and start depleting his HP rapidly. The aperture for the room is too wide to use for cover; what does the player do?

While it's technically possible to slip through the trip-lasers without detection, most new players aren't going to accomplish that feat. Instead, the player needs to quickly form a plan of how to get into cover, and where to go after that. The descriptor that tells the player how to do this is the shoot-through lattice highlighted in the screenshot. This screen won't stop bullets, but because it's connected to an obviously impermeable pillar, Freeman can't walk through it. The pillar on the right shows us exactly where the impermeable screen stops. The plan starts to spell itself out after this: get to the first box on the left, and then slowly move Freeman around the turret counter-clockwise.

It's not until after the player has a tentative plan that the designers start throwing in complications. The first big complication is a series of Headcrabs that teleport in one by one once Freeman has reached the second (right) cover boxes.

This is just the designer's way of teaching the player not to panic while in cover. This is a fundamental concept the player needs to know about cover. Standing and running will result in damage or death, especially since the next box ahead is actually an explosive crate, but staying relatively still will result in the turret shredding the Headcrabs as they overleap a crouching Freeman. This is a lesson that the player needs to know, and which this set piece teaches well: don't panic. The nature of cover is that it allows the player to wait for the right moment; rushing will only remove that advantage.

SET PIECE 5-2: Turret Evolution/Expansion

This set piece is a typical evolution/expansion of the last set piece. There are more turrets here, and there are several new elements with which they combine. The basic setup is the same: the player needs to get into cover and then slowly work Freeman around each corner to defeat the turrets. The trip-laser is also nearly unavoidable but for completely different reasons. Where earlier the trip-lasers were set up to cover the entire passage, this time the trip-laser is easy to jump over, but that's not going to help.

When Freeman enters the first cover location, two Vortigaunts will teleport in one-by-one directly behind him, which is a classic evolution of the previous set piece—an upgrade from Headcrabs to Vortigaunts. Without knowing beforehand that they're coming, it's almost impossible to stop them before they trip the laser and activate all the turrets. The designers were kind in that they accounted for a panicked reaction to the teleport, and placed two explosive barrels behind the Vortigaunt spawn. The explosion that kills the teleporting Vortigaunts is actually part of the descriptor mechanism for the room. In the center of the room there's another set of explosive barrels which can be used to disable the middle turret without exposing Freeman to fire.

Two explosions into the set piece, it should be clear that this set piece is all about triggering explosions (or just generally killing enemies) from behind safe cover. The player has recently picked up grenades, and this is the perfect time to use them, bouncing them off the walls or pillars to hit the turrets The two rear turrets aren't immediately dangerous because they're behind indestructible crates, but the confined space and height differential make them perfect targets for more bounced grenades thrown from above or through the small cracks in the crates.

If the player does choose to use grenades (instead of guns, which are effective from behind cover), the only really difficult part of this is learning the physics and making the throws before running out of grenades.

Segment 2: Architecture as Cover

The second segment of the cover theme pushes the player's ability to interpret cover by removing object cover and offering architectural features that serve the same purpose more subtly. Object cover does not disappear completely during this segment, nor does it ever vanish from the game, but it's less clear in later set pieces what is and isn't cover.

Most of this cover looks like ordinary walls and pillars; i.e., the kind of "cover" players might find in Quake or Doom. There are several important differences in both quality and quantity, however. In Quake, an enemy might chase the player character from one arena, down a straight hallway into another arena. There's no place to stop; everything is done by kiting and strafing. When there’s cover in Quake, it’s often an accident of the level design. In Doom, the player can make cover out of the terrain, but only because the enemies are slow and usually have a simple herd AI. In Half-Life, the enemies will act as individuals, shooting at medium range, charging at the player, and lobbing grenades. The player doesn't know what to expect, and as such, he or she can use the cover to buy time.

Half-Life set pieces are, to a certain extent, puzzles to be solved; cover is one of the pieces of that puzzle. In Doom and especially in Quake, the player has to start running as soon as the enemies aggro. By ducking into the first piece of cover available, the player gains a little bit of time with which he or she can look out on the rest of the set piece and plan the next move. Because the enemies in a Half-Life set piece aren’t usually in a herd, it’s actually possible to isolate certain enemies rather than take on the whole set piece at once. In the first segment, we saw how the first aspect of cover discipline was not panicking. The second aspect of cover discipline, which emerges here, is strafing around the corner of a piece of cover to create the most advantageous angle of fire. The right angle can cut down the number of enemies that can fire on Freeman while still keeping a few in sight to pick off. Especially in the second half of this segment, we’ll see lots of geographically large set pieces that the player can cut into smaller sections by using cover properly. Mostly, the player can use the same one or two pieces of cover for the entire set piece, so we haven’t yet reached point-to-point cover. In the next segment, the player will have to use several pieces of cover in sequence, but here the player mostly has to edge out a bit in one or two spots.

Cover descriptors are a bit scarcer in this segment and more subtle, although they do appear more than once. The left-hand hallway in set piece 5-3 can only exist for one reason—because the player should use it to get a different angle on the enemy in the back of the set piece. That’s a descriptor, although it’s a bit unusual. The barnacles in set piece 5-4 will eat enemy marines, and so serve as a kind of descriptor too, telling the player to simply stay away from the enemy’s line of sight and let the infighting do the rest. Laser trip-mines, making their first meaningful appearance in the player's arsenal, serve as an unusual descriptor for set piece 7-2, calling for their own use in the narrow halls of the surrounding area. None of these are as clear or as definitive as the cover descriptors of the first segment, but they do give shape to the set pieces they occupy.

SET PIECE 5-3: First Marines

The purpose of this set piece is to introduce the next evolution in enemy type: the HECU marine. Up until now, the cover set pieces have been based around stationary turrets or small handfuls of alien enemies, none of which present a particularly complex threat. The turrets do a lot of damage, but they can't move. The aliens can do damage and have a decent amount of HP, but their AI doesn't account for cover well, so it's easy to out-think them. The HECU marine is both highly mobile and (relative to the rest of the enemies in the game) highly intelligent. The difference in intelligence and speed becomes clear very quickly in this set piece.

There are two descriptors here at the very beginning that explain the use of cover and terrain. The first is the marine shooting down on the scientists from the catwalk above. This tells the player to use both the ceiling and the boxes as cover, and not to run out into the open. Doing this, however, will make it surprisingly difficult to actually kill the marine above as he dodges away. The necessity of the cover will become clear when a marine with no helmet will start charging, and will quickly stick his gun around the large crate at the end and start firing on Freeman from close range. It's very difficult to see him before he's already at close range, and so the easy answer is to shoot the explosive barrels on the right—after all, the player just did this in the last set piece a few minutes earlier. The marine’s fire may incidentally detonate these anyway.

The first half of the set piece is about how, even with cover, the intelligence and speed of the marines makes them dangerous enemies at close range. The second half of the set piece is about the difficulty of fighting them at range. This is where the explosive barrels come into play; the explosion does a great job of eliminating the charging marine, but it leaves Freeman totally exposed. It will quickly become clear to the player that the marines are still dangerous at long range, and the lack of cover is a real problem. There's an answer to this, though, in the level design. The left-hand path that had been partially blocked is now open, and provides an ideal place to hide from enemy fire.

From the aperture at the end of the left-hand passage, the player can get a very different read on the back half of the set piece. The first thing to notice is that the rear marines are definitely not charging; they're just firing from afar, although they do move laterally. Standing to the side, it's possible to see through the many pillars and boxes and get a few shots off with the element of surprise. This passage also takes Freeman a lot closer to those mid-range boxes so that he doesn't have to run through enemy fire for a prolonged time. There's no easy way to beat the rear marines—the explosive barrels aren’t placed near them—but by avoiding the center firing lane and using the boxes and pillars as cover, Freeman can close the distance and take them out one at a time.

As a historical note I want to point out how, to players accustomed to Doom and Quake, this set piece would have been absolutely astonishing. The enemies of Doom and Quake—and even shooters as far back as Space Invaders— never had the combination of intelligence and speed that the HECU marines do. There were plenty of enemies in Quake who would move quickly and charge at the player while firing, but none of them ever knew how to use cover. Moreover, this fight features marines with various behaviors. Some of the marines charge, while some fire at long range (although they do draw closer over time). In the days of Doom, this would have required two totally different types of enemies, but the HECU marine is more versatile. What's more, it was rare for single-player set pieces to mix up high and low enemies like Half-Life does, but it seems completely natural here. Since Half-Life came out, all of these things have become common to the point of seeming generic, but back in 1998, this set piece marked the beginning of a series of incredible shooter experiences.

SET PIECE 5-4: Above

For its level design features, this set piece belongs in the cover theme, and yet because of the way it operates (as a large battle with enemy factions fighting each other), it could almost be in the arena theme. There's meaningful cover in several locations, although there isn't a clear descriptor mechanism like a destructible object or an unusually high source of damage to be found. The important feature here is the presence of the barnacles all over the room.

Because the barnacles do not dictate movement or how to use cover, they're not really descriptors. Instead, they serve a different purpose. As you can see, the barnacles can and will kill several marines while they attempt to fire on Freeman. Before Half-Life, infighting between unrelated factions was uncommon in videogames. In fact, one of the common complaints made by reviewers of action games in the 1990s was that enemies were not subject to the same rules the player's avatar is. Enemies could walk through lava or move through fields of friendly fire without taking any damage. This set piece shows the player how that's not true for Half-Life; enemies are subject to the same damage and physics as Freeman is, and the Xen monsters will fight the HECU troops. The designers never have to explain it; the game simply demonstrates the fact in a clear way.

Once the enemies have thinned each other out, the player still has to be careful about proceeding through the long middle stretches of the level. Even with the infighting, there are usually some marines left roaming around, and they can shoot down on Freeman such that using cover is difficult.

This is just another exercise in using the architecture of the level design wisely. The marines here won’t throw grenades like they will in the next set piece, so the cover is meaningfully protective. The player can’t really fire back until Freeman can get the high ground. Once the player can get to the high catwalk, the catwalk itself will provide all the cover the player needs to finish off any remaining enemies.

SET PIECE 5-5: Below

This set piece opens with an introduction to the astonishingly accurate grenade-tossing skills of the HECU marines. Immediately upon entering, the player will have to run quickly to avoid a barrage of grenades which the marines can launch from below with great accuracy (and at a bizarrely slow speed).

After an attack of grenades that lasts for quite a long time, the set piece proper commences. The defining feature of this set piece is that it reverses the dynamic of the previous two set pieces completely. Whereas the previous two set pieces gave Freeman cover and the enemy the high-ground advantage, this set piece gives Freeman the that position and the enemy most of the cover.

All of the object cover belongs to the enemy, but the architectural cover is usable for Freeman. The support pillars work to deflect fire and can even be used to perform the standard pop out/in maneuver. The catwalk itself can also serve as cover from fire coming from below, although this requires that the enemy is out of grenades, and it is a lot harder to shoot from behind the catwalk than a box. Still, Freeman can basically stand in one place and pick off all of the marines because of the height advantage and the enemy movements.

SET PIECE 7-1: Ladder to Hell

Here the player takes Freeman up a ladder into a highly deceptive hallway encounter. This set piece is a reversal just like set piece 5-5; the enemies have the advantages of cover this time around. This set piece is much shorter than the previous one in the theme, though, and has a relatively low enemy population. What it lacks in length and numbers it makes up for in trickery. After climbing the ladder, the player is faced with what appears to be a standard corner turn. The far wall on the left is visible and suggests a dead end with no threat. The hall on the right clearly goes further.

The obvious threat is definitely the two marines on the right who charge right at Freeman. There's not much in the way of object cover; the player has to use the corner to pop and fire. The marines are so close, however, that their aggressive charge will negate this cover pretty quickly. The game has shown this behavior enough by now that the player might know this and simply start strafe-firing. This is where the real trick of this set piece kicks in. That apparent dead end on the left end isn't a dead end at all; it's a hidden-enemy corner. In that corner is a turret which will fire on Freeman from behind while the marines shoot at him from the front.

Hidden-enemy corners are a design trope going all the way back to Wolfenstein. In order to serve their purpose, they need to be surprising, and so they’re usually placed in an irregular and apparently non-threatening place. This spot is irregular, as the left hallway isn't flush with the right hallway, exposing Freeman to the marines first. It's also apparently non-threatening as the player’s first glance suggests that hallway is a dead end. All of that is just a calculated deception. The only thing that keeps this set piece from being totally unfair is that the turret doesn't sweep the whole hallway, only the left side of it (facing down toward the marines), and the angle is such that it can't continue firing on Freeman for very long if he runs through quickly.

SET PIECE 7-2: Behind Every Door

This set piece is one of the most difficult in the game to classify. I consider it to be one set piece, but it could really be two or more. All of the marines in this area come in groups of one or two, and aggro always depends upon a different criterion in each case. There are two locations where most of the fighting takes place, but there’s a high degree of variability in how the enemies will approach Freeman when the fighting begins. The overall purpose of this set piece (again, if you can call it that) is instruction in the tactical uses of the laser trip-mine. Trip mines are not very useful if a huge line of enemies is approaching, because they only kill the first one or two out of a huge group. On the other hand, they're too useful if, for example, five Vortigaunts all charge together. So the piecemeal pairs of marines in this section make sense because they are just right for trip-mine play. The level design also has some unusual features which make sense for trip-mine placement and little else.

The initiating incident for this section is a novel take on the concept of binary cover. The first marine in the section hides behind sandbags, while Freeman has some aperture cover. Behind both Freeman and the marine are explosive crates.

Essentially, these crates work as the opposite of binary cover; instead of shielding both Freeman and the enemy from fire they put both of them at much greater risk. By beginning with an explosion, however, the designers have reminded the player that explosions are a very effective way of dispatching marines. The obvious placement of laser trip-mines also tells the player what needs to be done: either arm a mine in the doorway or along any of the protruding walls just inside the doorway.

The doorway is narrow and makes for good cover. The hallway beyond the door features several protrusions that also offer decent cover. So whether the player uses mines or starts shooting, there’s cover available to do it. The real problem is that enemies are going to come from both the left and right. They don’t come all at once, but taking too long to kill any one group will result in Freeman being surrounded. Thus, it’s clear the designers intended the player to have at least some part of this set piece protected by an exploding mine. The time will come later when the player faces a whole segment based around being surrounded, but the game is not there yet. There’s no architectural provision (nor appropriate amount of training) for dealing with combat on multiple sides; there are only mines and lots of re-attempts.

SET PIECE 7-3: Ambush

This set piece reverses set piece 7-2 in that the player starts at the bottom of the elevator shaft and has to fight out against marines who have entered by the same path the player took only moments before. The only surprise the player faces here is the elevator which drops a few marines on Freeman at the beginning of the action.

After that point, everything in this set piece is about as standard as it could possibly be, with Freeman having lots of easy-to-use cover. Indeed, everything that was once an advantage for the enemy is now available to the player.

Although there isn’t much to say about this set piece, I want to point out that in every genre, it’s important that not every consecutive challenge be bigger and more difficult than the last one. An endless uphill battle is bad for player morale and stamina; the up-and-down motion on the axis of obstacles that Space Invaders invented is the core concept of videogames for a reason. This set piece is an example of the designers scaling back the difficulty to give the player a break.

SEGMENT 3: Point-to-Point Cover

This segment begins to expand the player’s cover discipline abilities by forcing him or her to move from one piece of cover to the next while under fire or threat of fire. Obviously, this segment is not the first to make the player move around to take advantage of cover, but it does increase the complexity and danger of doing so. In the last segment, most of the player’s movement was a retreat into or around one or two pieces of cover. When the player goes into the tunnel to get a new angle on the marines in set piece 5-4, that maneuver was still a retreat into very secure cover. When the player backed out of the door to use mines in set piece 7-2, that too was a retreat into safety. In this segment, the player is charging toward enemies and/or across dangerous open spaces to get into the right cover. What makes that cover “right” is that it gives the player the most advantageous angle of fire. Scooting around behind cover won’t give the player enough of an angle; Freeman has to move through the dangerous spaces.

The best example of an advantageous angle of fire is the accessory spaces in the second set piece of this segment. Set piece 8-2 starts the segment off by giving the player small rooms on either side of its main corridor.

The player can draw the aggro of the enemies and then duck into these rooms, whose small apertures will protect Freeman from extra fire while he shoots his target. The accessory space is just the most extreme example of a trend that gets progressively more difficult here. Crossing between two pieces of cover on either side of an enemy group is a theme that sees a lot of development.

The player has to do this same motion through several more set pieces immediately after 8-2. In those set pieces, there is no cover descriptor, although each set piece is essentially the same cover discipline puzzle presented in a different way. There are alternating groups of HECU and Vortigaunt squads occupying various passageways in this area. To conserve health and ammo, the player has to find the right cover and best angle to break these groups up into smaller parts, and then deal with those broken pieces from behind cover.

These later set pieces are actually related to each other in a classical cadence style. Each set piece becomes more and more dangerous as the “best angle” cover gets shallower and harder to spot. In set piece 8-3, the accessory spaces disappear and the dangerous passageways start intersecting in more complex ways. In 8-5, the cover offered by the few nooks in the wall becomes a lot shallower, and the number of enemies goes up. The end of that set piece culminates in two spot-checks with marines firing high-caliber cannons from inside small bunkers.

This is the final test of knowing when and how to zig-zag, and it's considerably more difficult than any of the other iterations because the cannon can kill Freeman quickly. The strategy for all of these set pieces is essentially the same, however: maintain cover discipline by always choosing the angles that both grant safety to Freeman and an angle that reduces the number of enemies that he is firing on at any one time.

SET PIECE 8-1: Parallax

Enemy infighting returns in this set piece and remains a prominent part of the design for the rest of this segment. The next few set pieces are built around three-way fights between Freeman, the HECU troops and Vortigaunts. Although the number of enemies in this set piece and the next few is high, one important thing to observe is that the enemies don’t operate as one herd, and this battle is the largest of them all in terms of enemies on screen at the same time. The set piece is set up as a three-group brawl intersected by two electrified cart tracks.

The marines don't usually move more than a few steps, and will mostly remain in position and fire at the opposing Vortigaunts. The Vortigaunts also don't cross the cart tracks, but will move around more in order to get a clear shot off at the marines. There is cover in the set piece, but no clear descriptor about how to use it. The marines and Vortigaunts don’t use the cover in an obvious way, and there’s no other mechanism (like one enemy more powerful than the others) to show the player where to crouch for pop-in/pop-out firing. That's not to say that the set piece has no flow; it does. It's much better for the player to pick one faction to destroy first, whether starting from the back and killing all of the marines, or starting from the front and killing all of the Vortigaunts. Either way, it's a bad idea to get caught in the middle of the two factions. They will shoot Freeman to death quickly, since the cover can only block shots from one side. This isn't technically a descriptor though, as knowing this fact involves seeing the entire set piece. Descriptors are only useful when they are able to communicate the flow of a set piece at a glance. If the player has to see the whole battle to understand it, as in this set piece, the purpose of a descriptor is lost.

With this in mind, there's still a lot to know about how this set piece works, and how it relates to the numerous set pieces which follow closely on its heels. Although it isn't immediately apparent, this set piece features an hourglass/chokepoint design. On both ends of the conflict, it's possible to move around a lot, but the middle is quite confined.

Not only is the middle platform narrower than the outer platforms, it features object cover on both sides, so that the player can strafe to get the best firing angle, especially on the Vortigaunts who are spread out behind pillars. You can see on the map how this would work. The set piece is much easier for players who are trying to go back to front, killing the marines first, and the flow makes a lot more sense. Dashing between the two central pieces of cover is not only more useful when fighting Vortigaunts, but also safer. The Vortigaunts aren't good at firing around cover, and they never have grenades. No matter which way the player chooses to approach the set piece, however, it's clear how the flow of the action works, despite the lack of a cover descriptor.

SET PIECE 8-2: Zig

This set piece looks at the same problem as the previous one with a slightly different setup and fewer enemies. At first, maps of these two set pieces look entirely different, but the way the action works is actually quite similar. In each segment of this hallway there's a new set of enemies that is unattached to the other enemies in other segments.

The big addition here is the two accessory spaces in each segment of the room. These are surprisingly similar to the accessory spaces we saw in set piece 4-3—even to the point where the player is facing Vortigaunts again. There are some marines mixed in as well; this can be seen as an evolution of the Vortigaunt-only ancestor. The accessory spaces are perfect for luring in Vortigaunts, and adequate for luring in marines. To complicate things, the first accessory space actually has a marine camped out in it, and he'll stay there even with gunfire blaring outside his room. The second room doesn't have this problem, and is quite a bit bigger. Like the earlier accessory spaces, these rooms also double as troves for loot crates.

Looking at the overall map flow shows us something familiar here, though: the same kind of forward-and-across motion that we saw in the last set piece. Really, this can be seen as an evolution of that previous set piece, in terms of space.

What was once a quick motion across a chokepoint is now a larger motion across an open corridor with accessory spaces attached. The player has to create the chokepoint by luring the enemies into the apertures of the accessory spaces. That's a qualitative evolution for sure, although it also has a corresponding drop in the number of enemies to offset the increased complexity.

SET PIECE 8-3: Zag

This set piece features a fairly obvious evolution: there are two different pathways occupied by groups of enemies, and they intersect. Like set piece 8-2, there's more than one way to go about dissecting the groups; the player can loop up around to knock out the distant Vortigaunts, or can lure the marines beyond into a three-way firefight, although starting such a fight is actually much harder than it sounds.

(Transparent cone represents firing angle)

No matter how the player disposes of the first two groups, the third group in the back/right is best engaged from the jutting corners in the hallway. These corners are actually a bit shallower than in the previous set pieces, which is a kind of expansion challenge (the “expansion by contraction”). The contraction of cover is a form of quantitative change in an already-established design idea that makes this set piece harder than its parent; thus, it’s an expansion. Like in the last set piece, though, there aren't too many enemies, so it's not a huge problem. Once the player has neutralized the group of Vortigaunts on the left, it's a fairly straightforward fight.

SET PIECE 8-4: Three Sentries

Whether the player has the foresight to get off the cart before hitting the end or not, this set piece starts off with a difficult angle; the three marines have a height and numbers advantage in addition to their possible surprise advantage. Hiding behind the cart works somewhat well. Running into the tunnel to the right will actually put Freeman in a new set piece, which vastly increases the overall danger, and so is not really an option.

Players who are quickly shredded by the overlapping fire zones of the marines may try to stop the cart and approach on foot. The angle on the marines at the beginning is still quite tricky; there's no jutting wall to protect Freeman, and the three marines will always aggro at the same time. Moreover, the elevation of the platform where those marines are standing makes it very hard to find a good angle of fire, as the enemies have an increased ability to fire from their advantageous height. The only advantage the player has is that there’s only one stairway, and the three marines will sometimes stack themselves up if the player hides in the tunnel after aggro. This is similar to the earlier accessory-space trick, although the player may well have to spend a few deaths and replays in order to pull it off. Although this is not objectively the most challenging set piece in this segment, it is the least transparent in terms of player strategy.

SET PIECE 8-5: Deadly Corridor

Because the previous set piece lasts long enough to be considered extended, and because it shares no aggro triggers with this set piece, I consider them separate. One could certainly make the case that set pieces 8-4 and 8-5 are one large entity, but I think the two parts of them are also different enough that studying them separately reveals more about them than studying them together. This set piece is another corridor with jutting walls that allow the player to go point-to-point again to get the angle on the marines and Vortigaunts.

(Blue circles and cones show player firing areas, while the red cone shows the cannon’s area of fire.)

Each bit of alcove cover is quite shallow now, which continues the trend of expanding-by-contraction that has lasted since set piece 8-2. This expansion is reiterated on the back half of this set piece when the player has to make short dashes between boxes as a high-powered turret fires on Freeman.

Here the damage has expanded while the alcove size stays the same. (A change in alcove sizes doesn't really matter against a stationary enemy who can't charge around that corner.) Much like the turrets of segment 1, this cannon emplacement has a slight delay in firing, but it deals so much damage that unless the player is an absolute artist with grenades, Freeman is going to take substantial damage. By making the fastest possible dash from cover to cover to flank, though, the player can get through without having Freeman die.

SET PIECE 8-7: Rest Stop

This set piece actually offers two paths of ingress, both of which offer decent solutions for getting the cover angle which will give the player a tactical advantage. One of the paths requires moving past the line of the Vortigaunts’ fire, but this is usually done while on the rail cart, whose speed minimizes Freeman’s exposure.

The player can even lure the Vortigaunts around the pillar in a loop, picking the best angles at the best times.

After the first encounter with its two paths, the rest of the set piece straightens out somewhat blandly. There’s another corner with some more Vortigaunts to draw into a cover trap. Still, the most interesting part of this set piece is definitely its front half, where the two different approaches give the player some room to be creative.

SET PIECE 8-8: End of the Line

The cover descriptor returns in force for this location. In this incarnation, the descriptor takes the form of a powerful cannon set directly in the path that Freeman needs to take. To some degree, the raised walkway also acts as a descriptor, shaping the player's movements, but it's the cannon that really shows the player that they can't simply walk in and shoot freely.

This set piece also does a number of other interesting things. The first thing to notice is the return of object cover—and plenty of it. This is the first set piece in a long time to provide so much of it, although all of it belongs to the enemy, unless the player can get Freeman up the ladder without dying. The second thing to notice is that the marines in this set piece is that they are scripted to only move around behind the cover. Indeed, this set piece is a total inversion of the current segment theme. The enemies are executing point-to-point cover maneuvers. Accordingly, the player has two options: the first is to engage from the aperture, ducking in and out and spending lots of ammo to destroy the crates and the marines behind them. The other option is to duck under the lip of the walkway for cover. In a sense, this is the inverse of what has been going on in this segment so far. Instead of gaining the best angle on the enemy, the player can simply negate the enemy’s angle on Freeman. This gives the player a chance to use grenades to disable the heavy cannon on the right before charging into the cover to battle the marines from a shorter distance. Both of these options are difficult and expensive (in terms of health/ammo) relative to the rest of the set pieces in chapter 8, but they're both definitely viable strategies.

SET PIECE 8-10: Two Cannons

This appears to be an exceptionally short set piece, but because of how slowly the player has to proceed through it (or how many deaths and reloads it requires), I think it passes the “extended” criterion just fine. The player faces another gun emplacement, this time accompanied by one of the elite, helmet-less marines. The elite marine is more aggressive than the normal marine, and uses the grenade launcher on his assault rifle fairly often. But no marine in the game is more likely to use the grenade launcher than this individual soldier.

Although both guns could easily destroy the cover available here, they won’t do so. Thus, the player is free to slowly dash between pieces of cover to get the best angle, or even slowly slide some of the crates forward until Freeman is close enough to make a devastating attack of his own. Either way, the cannon emplacement is difficult to damage and both it and the elite marine are very deadly, and so this set piece takes much longer than it looks like it should. For all this difficulty, however, it serves as a good final challenge to the point-to-point cover segment.

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