This level occupies the map slot MAP15. For other maps which occupy this slot, see Category:MAP15.
At the map start, go outside to the western part of the balcony and look to the west. There are two stone lion faces in the cage: shoot at the northern (right) one to remove the bars in the northeast of the balcony. (Shoot the southern one to lower the pillar with the blue armor near the bars.) Go to the northeastern part of the balcony and drop down into the blood lake. Ascend the stairs in the southwest of the aforementioned balcony. At the top of the stairs, keep going west two more steps. Right in front of the steel beam, turn left. Shoot at the bloody spot in the southern hole to raise the ceiling above the candle in the niche in the north. Step on it and shoot another bloody spot in the hole in the southeast (to the right of the lion face) to raise the aforementioned steel beam (sector 393), revealing a computer area map. (Pushing the lion face reveals a berserk pack.)
Run from the south through the hole in the wall where the yellow skull key is in to jump over the spiral walkway and land on the linear walkway leading to the north. Follow it and enter the western staircase at the fork. Go through the door and descend the stairs to your left. Push the lion face behind the green armor to lower the surrounding walls. On the top of the lowered walls are three torches, each on a triangular stone. Go to the east of the southern triangle and look to the west. You have to straferun over the southern angle (do not run into the torch as it will stop your attempt) and over the wall to jump onto the first "step" (sector 531) of the bloodfall in the west to get the soul sphere in its north.
After acquiring the red skull key, pushing the lion face on the platform in the south of secret #2 removes the red bars in the building west of the platform (and east of the building where the map start is). Enter the building through the short hallway from the west as you did before and instead ascending the stairs in the north, look to the south. (If you unsure about where the red bars where before: On the left and the right sides of the southern stairs are altogether nine health bonuses and nine armor bonuses.) Run up the stairs and through the cross-shaped hole, jumping over the wall in the south to enter an area unreachable before. Immediately run to the west, around (or through to pick up some armor bonuses) the southern side of the "8"-shaped (look at your map) structure in your way. On the western side (where some demons and spectres await you), run up the steps in the blood lake. To the north of the apparently uppermost step is a bloodfall. Walk through it to find a hidden place (sector 303) with an invulnerability at its northern end.
Step on the "8"-shaped structure mentioned in secret #3. Ascend the stairs and push the switch on the northern side of the support beam in the north of the tower to raise the stairs behind you. Go upstairs and approach the center of the 8's southern circle from the north to open a teleporter. Do not enter it, instead walk around it on the narrow ledge. Push the switch on the southern side to raise some ledges near the map start. Go back to the north of the aforementioned support beam. Simply walk to the north over the gap to step on the wall surrounding the map start building. On the wall, walk to the east over a newly raised ledge outside the hallway mentioned in secret #3 to get to a platform (sector 871) with a backpack.
Push the switch in the northeastern corner of the platform from secret #4 to raise a small step in the west of the newly raised ledge north of the hallway mentioned in secrets #3 and #4. Use that step to walk over that ledge. On the platform in its east, look to the north. You see a BFG9000 in a elevated window east of the yellow door. Just walk over the wall pointing at it and go through the fake wall below the window to lower the pillar with the BFG on it. You have to step on the ground (sector 634) (where a stimpack rests on) to register the secret.
From secret #5 go back to the platform in the south and walk over the ledge in its northeast to reach a platform (sector 204) with a megasphere on it.
Enter the building in the southwest of the "8"-shaped structure mentioned in secrets #3 and #4. Descend the first set of stairs, go left (west) and then right (north). From the northwest of the round room in the north of the building, open its wall under the gap and step on the candle in the middle of the blood pool below the hanging corpse to open up a door in the south of the mancubus's alcove in the southernmost part of the building. Go outside and head back to the 8-structure. Use the center of the southern ring's northern part as a lift. Use the teleporter you were told not to enter in secret #4 to get to the mancubus's alcove. Walk through the southern door to the ledge (sector 772) southwest of the stairs leading to the building's entry where you find a medikit.
Secret Exit: Still on secret #7's ledge, enter the door in the southeast and go right (south). Use the teleporter to the yellow key room, and run straight forward to the north as you did before. This time, two pillars have been raised from the ground right in front of the door you were unable to reach before. Walk through the door and drop down. Step on the red hexagon to lower some floors. Go east over the bloody ground, enter the building and the now reachable opening in the south. Follow the stairs to find the room with the secret exit to map MAP31: Cyberdemon Vertigo. (Note that the room itself does not count as a secret, but the uppermost bloody stair (sector 766) in its south.)