This level occupies the map slot MAP23. For other maps which occupy this slot, see Category:MAP23.
From the map start, push the switch in front of you and take the western route. As you arrive at the teleporter in front of the giant mud lake, look to the northwest. The second (and smaller) mud fall from here is passable. Run through it, to the back end of the cave behind it and into the hole that opens in front of you. Approach the evil eye to be teleported to a dark area (sector 224) in the very south of the map. There are a spider mastermind, four medikits, eight energy cells and a BFG9000. Use the teleporter in the north to get back to the map start.
After pushing the lionhead switch downstairs in the south of the blue skull key and defeating the ambush, you have to use the gargoyle switch on the western room's northern wall to remove the northern bars on the eastern wall. In front of this gargoyle switch, look at the western wall: in the south of the northern green torch is a rectangular recess in the wall. Shoot at it to open the canvas and enter the new hallway to find a stimpack, a set of shells and a chaingunner. Go north to register the secret (sector 402) and find a soul sphere, a bullet box and a backpack.
From the red skull key run to the east, jump over the island with the green torch and from there into the wall's large opening (sector 131). In the hallway you find a bullet box and a megasphere.
After using the switch in the north of the yellow skull key and exiting the room through the southern door, you see that the wall in the center of the semicircular stairs has lowered. In the new area, open the recess in the eastern wall and approach the evil eye to be teleported to a dark area in the very north of the map. Leaving the destination teleporter lowers the wall in front of you, revealing a nasty ambush: There are two arch-viles and many (10 on easy, 20 on medium, 30 on hard skill levels) revenants. Grab the invulnerability sphere to deal with it. There are some more useful items around: two bullet boxes, six shell boxes, two energy cells (and six more on hard skill), four bulk cells and eight medikits. After killing the enemies, use the switches south of the arch-viles' platforms to open the doors in the north. Walk towards the soul sphere to register the secret (sector 582) and approach the evil eye to be teleported back.