This is an underground city filled with all the strong enemies. You will not find weaklings - they simply didn't survive. Also, look for the Cyberdemons' revenge: remember doom2 level 8 (Tricks and Traps)? Now's the Cyberdemons' time for payback!
This level is so hard because Haggay got angry after 29 turned out too easy :-)
Difficulty: very hard
In the eastern section leading to the yellow door there is a demon-face on the northern wall. Press it to lower an elevator to a secret compartment containing a BFG 9000 and a Bulk cell.