This level occupies the map slot MAP28. For other maps which occupy this slot, see Category:MAP28.
After stepping on the black platform with red lights where the blue skull key and the cyberdemon rest on, you see that a wall lowers behind the fence in the east, releasing some monsters. Return to the room (with the ceiling that looks like a wire wheel) in it the southwest of the platform. Enter the new passage in the southern wall that leads to a room (sector 424) where you find a berserk pack, two stimpacks, ten health bonuses and a blue armor. The switch on the eastern wall opens the nearby bars.
Return to the map's central intersection (that is exactly in the east of the blue skull key and in the north of the red skull key (and the map start). Enter the eastern door and then the southern or eastern entrance to a circular cavern. As you enter the cavern's center (where you see a switch) from its south, you trigger two small caves in the west and east to open, releasing some revenants. Follow the passage in the north of the western cave that leads to a room (sector 336) with a soul sphere.
From the north, run to and over the center of the platform where the yellow skull key is on and to the south into the teleporter. Run to the northern wall and enter one of the four temporarily lowered lifts before those rise again. After riding the lift up, push the lion face to lower all lifts again. Quickly run to the alcove in the north of the area where the lion face is that removes the bars blocking the area's exit. There are two more lifts, enter the eastern one (sector 197) that contains a soul sphere. (On hard skill levels, each one of the six lifts contains a revenant.)
Ride up the lift in secret #3 and push the lion face to lower all lifts one more. Enter the western one (sector 198) where a blue armor is in.