Right from the start, go in the small blood pool, turn right and hit the switch (linedef #5142): it will open two rooms around the Mancubi, get the berserk, the megaarmor and other stuff.
Follow the big blood pool of the third screenshot. You will get hurt, find the teleporter. There, hit the switch, kill loads of spectres with the BFG 9000.
It's not really a bug, but only one of the soulspheres that lay near the blue or red keycard can be grabbed, so that you can not finish the level with 100% items. When you approach one of them, the other one is upped on its pillar.