This level occupies the map slot MAP20. For other maps which occupy this slot, see Category:MAP20.
From the map start, go to the south. In the large room, where the path splits into the western and eastern route and you find a plasma gun, use the teleporter north of the mancubus's platform in the northeastern blood pool. After teleporting, turn around to the north and open the wall to enter the place (sector 248) east of the northeastern blood pool with a backpack, a medikit and seven health bonuses.
In secret #1, look to the northwestern corner of the northeastern blood pool. There is a green rectangular recess on the steel beam's eastern side. Shoot at it to open a door in secret #1's eastern wall. Enter the new discovered room (sector 318) to find a BFG9000.
From secret #1 go through the passage in the south, drop down on the walkway and into the southern blood pool. Enter the red teleporter in the northeast to reach the place across secret #1 in the west of the northwestern blood pool. There are four rockets, a medikit and seven armor bonuses. Open the northern part of the western wall to find a niche (sector 1219), containing a partial invisibility.
From secret #3 jump on the mancubus's platform in the northwestern blood pool to remove the bar in front of the niche in the north of this blood pool. (Jumping from secret #1 onto the mancubus's platform in the northeastern blood pool has the same effect.) There is an invulnerability sphere in the niche (sector 355).
From secret #4, use the nearby teleporter to get back to the main path. Follow the eastern route. After two bends to the north and to the east, you enter a semicircular hallway. Shoot at the wall behind the first (southernmost) red torch behind a mesh to remove the bar in the north of the skull door in the east of the semicircular hallway. Grab the berserk pack there to register the secret (sector 1190).
From secret #5, pass through the nearby skull door. Push the switch behind the blue armor to open the bars blocking the semicircular hallway's northern exit. Enter the passage in the northeast or southeast of the room and ascend the stairs to find a switch to reopen the skull door so you can exit the room. Exit the semicircular hallway to the north and follow the path to the west until it makes a bend to the north. Take a good inrun and jump on the higher part of the blood floor westsouthwest of the southwestern corner. Walk to the south behind the fence to find a niche (sector 1182) with a soul sphere in the western wall.
After removing the blue and yellow bars, keep moving westwards. Use the two switches near the backpack and return to the room south of the starting area with blood pools to the west and east. Walk over the new bridge with the gap to the east, but do not drop down to the hallway with the BFG-9000 yet. Instead, jump over to the northern ledge where some imps are. There is a recess in the wall to your right at the ledge's end. Open it to reveal a switch. Push it, drop down to the north and follow the now accessible passage to the east. Step on the safe floor (sector 151) in the room where you end up. There are two medikits and a teleporter. Use it to be teleported to the soul sphere across the aforementioned bridge. Immediately turn around to kill the arch-vile behind you. Use the teleporter in the north of the western blood pool to get back next to the starting area.