This level occupies the map slot MAP17. For other maps which occupy this slot, see Category:MAP17.
From the map start, leave the cave. Enter the cave in the north of the open area and push the northern side of the switch to open two alcoves (sector 50 each) next to the switch, the western one containing a chaingun, the eastern one a backpack. Enter either alcove to register the secret.
Ascend the stairs in the north of the plasma gun. Push the eastern lion face switch to temporarily open the fake gargoyle switch in the north of the western stairs. Quickly run through the opening to find a secret room (sector 192), containing two clips, two rockets, a berserk pack and a computer map. Walk trough the eastern bloodfall to find a switch. Use it to enable the later access secret #5.
Exit secret #2 and push the skull switch on the northern wall between the two stairways. Cross the bridge in the south and note the wooden cube with skulls on it to your right. Go to the blood lake to the east: the bars in its north have been lowered, enter the room behind. As you walk through it, the lift front of you lowers (and rises) simultaneously with the aforementioned wooden cube. Run there and go through the passage in its south. Enter the teleporter to reach the platform (sector 112) with the soul sphere.
Ride up the lift mentioned in secret #3. In the northwestern corner of the hallway is large window, outside you see a small water area. From slightly west of the window's eastern edge, look to the northeast of the water area. There is a pair of red glowing eyes in a wall's recess. Shoot at those and look at the southeast after you heard the door opening sound: a passage (sector 264) around and in the southeast of the room you are in has opened. There are 13 health bonuses, a bullet box and a bulk cell.
Exit secret #4 to the north. Follow the hallway to the east and around the corner and fall down into the hole. (Opening the door in the south of the blood lake area you are in opens a part of a wall blocking access to a teleporter.) Proceed until you arrive in the hall north of the circle where you will have to use the keys. In the northeastern part of the hall, northwest of the until now unreachable BFG9000, is a pile with skulls and candles and a dead marine. If you pushed the switch mentioned in secret #2, there is also a opening in the wall. Enter the teleporter in the room behind the opening to be teleported to the platform (sector 459) with the BFG on it.