Near the beginning of the level is a large, black-walled room with several desks in it. In this room is a teleporter pad; step through and you'll be taken to a nearby chamber with some crates in it. There is a series of small crates leading up to the top of a large wooden box with some shotgun shells on it. Open the wall of the adjacent crate (it has a "3" on it) to reveal a secret area. One of the walls inside this secret can open, letting you exit.
Accessing secret #1 will cause one of the walls in the crate room to lower, revealing some armor and a computer map. The outcropping near these items has a red brick texture; lower it for a medikit. Ride the lift back up and you'll reach a secret area with a BFG. While standing at the top of the BFG's pedestal, you will see a gargoyle face on the wall far to the south. Shoot it to lower the nearby gate, letting you exit through the teleport in the gargoyle's alcove.
In the nukage pool area containing the blue skull key, there is a metal-textured wall to the west. Open it and pass through the long tunnel (the seeming dead end is in fact a lift). You'll eventually come to a room with lava in it; the second tunnel leading out is tagged secret. It's impossible to complete this level without triggering this secret.
The second half of the tunnel from secret #3 is also tagged secret and impossible to miss.
Upon entering the room with the yellow skull key, you'll see the long, diagonal wall to your left lower before you can reach it. Simply open it back up to get the bulk cell it hides.
Just around the corner from secret #5 (directly across from the yellow skull key itself) is another openable wall with a slightly different texture from the ones surrounding it. This one holds an invulnerability sphere.
At the end of the level is a huge outdoor area with a number of fenced-in ledges surrounding a long river. While walking along the ledge, you'll have to take a lift up to the next ledge. If you ride it all the way to the top, you'll see a small alcove with a bulk cell in it. Open the damaged-looking wall in this alcove to reveal a teleport that lets you skip a decent portion of the ledge-traversing part of the level.
It is possible to get stuck permanently if you straferun from the BFG's pedestal into the barred corridor in secret #2 before you have shot the gargoyle face. Since the gate blocks your path in the north and the face lies too high to be hit from the corridor, you can only load a saved game, kill yourself to restart the level or use the no clipping cheat to make an escape.
Not so much a bug in itself as an opportunity to see one, collecting the Soul Sphere on the damaging floor in the outside courtyard is a great place to see the ouch face bug.
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Added by Ducon
Added by Fredrik
