At the beginning of the level, turn around walk to the door with lava in front of it. Open it, and turn the southern bend. Destroy the barrels, flip the switch, and go to the northern hall with the torches. Quickly flip the switch before the wall closes. Return to the beginning of the level, and open the door with water in front of it. Enter the northern bend, and then swiftly run into the southern hall to grab the red key before the wall closes. After this, return to the starting area and open the door with blood in front of it. Go through the red door and head down the stairway. Take the elevator down, and open the door.
Run toward any of the medikits in the middle of the room to reveal armies of sergeants and revenants on either side of the room. Dispose of them all, and align yourself with the middle wall of the north side of the western structure. Turn to the north, and open the northern wall to find a teleporter. Use it, and open the middle set of computer panels. Enter the teleporter, and then run straight to the other end of the walkway. Open the southeast wall to find another teleporter. Go inside, and open the misaligned wall in the next room to find the blue key. Open the eastern wall to go back to the western walkway, and then return to the eastern one. Open the northern set of computer panels, and you will find a lava cavern with a curving bridge. At the other end, align yourself with the northern side, and do a running jump to the walkway. It will take quite a bit of practice to pull this off, and it is significantly easier if you straferun. Grab the yellow key, return to the start of the bridge via the teleporter, and head to the yellow door to the north. Open it.
Go through the blue door to the east, and head down the stairway until you find two large alcoves at either side of you. Enter the eastern alcove, and go down the three successive lifts that follow. You will now be in a cage with a satyr panel in the eastern side, and a pair of doors and switches to the north and south. Align yourself with the satyr face, and walk toward the cage bars; this section of the bars is fake, and you can pass through it. Press the northern switch, and run into the teleporter behind the southern door. You will be taken into a curved lift. Press the switch to be taken down to the next room, and then press on the demonic faces to the west to reveal another lift. Run inside to be warped to another such lift, and flip the switch to be taken to a dark walkway to the south. Go all the way around, and flip the switch. Go to the east of the walkway, and run down the stairs. Open the door, and flip the switch.
Other points of interest
In the cage room, press on the satyr face, and then use the teleporter in the northwest. Look to the west to find a backpack. (sector 216)