In the red door section, after getting the blue key, in the southwestern corner of the corridor where the chaingunners came out, use the southern wall to reveal a secret niche containing a blue armor.
After entering the blue door, take any of the two corridors and descent the stairs. As you leave the last step, a secret room across the blue door opens temporarily. There are a bulk cell, a box of rockets, two medikits, a soul sphere and a blue armor.
After entering the blue door, take the left (northern) corridor, descent the stairs, make an hairpin turn to the left, enter the room with the teleporter and the monster hideout. Push the demon-faced wall to reveal a switch. Using it will open a stairway in the southern teleporter room that leads to a secret room, containing twelve health bonuses, 16 armor bonuses, two backpacks , two bulk cells and a teleporter to the map start.
After passing the yellow door, on the narrow staircase on the right side of the room with the Hell knights, push the wall with the lantern on it to reveal a secret room, containing three rocket boxes, two energy cells, a bulk cell and an invisibility sphere. Push the face on top of the stairs at the northern wall to access to the megasphere you saw in the crate depot early in the level.
On the opposite narrow staircase, using the corresponding wall will reveal a teleporter to the BFG9000 standing on the pillar in the centre of the main room.
For the same reason, some armor bonuses in the teleporter rooms (sector 395) west and east of the hallway exiting the BFG9000 room cannot be picked up.
In the south of the BFG-9000 room, there are two alcoves each on the western and eastern side of the exiting hallway, each containing two armor bonuses that are not tagged as multiplayer only. Those alcoves can only be accessed in deathmatch mode, using the idclip cheat or by using a source port that allows jumping.
This map shares three commonalities with its predecessor, MAP06: Baron's Lair of The Plutonia Experiment: both levels (1) are arranged with the player starting in a central "hub" sector surrounded by four doors leading to surrounding sectors, (2) use the music track "Sign of Evil" from the Classic Doom soundtrack, and (3) mark the player's first encounter with a Cyberdemon in the episode. Note that this last commonality is only the case on the UV and NM skill levels. When playing on HMP the player will encounter the Cyberdemon for the first time in MAP14: Charon Eclipse, and players who play on ITYTD or HNTR will not mark their first encounter with a Cyberdemon until MAP17: Nuclear Horror, or until the secret levels.