Siege entities for BSP.

Asuming BSP is installed in C:\BSP:
Unzip this to C:\BSP\entity. Run setup.exe in C:\BSP. Choose Hexen 2 when asked what game 
you want to set up. Look for default .qc(s) near the bottom. Change h3ents.qc to Siege.qc
then click Finish.

Make sure the spawnflags for worldspawn is set to "Siege" or your level will not work in Siege.