Enter the red brick building ahead of you via the lift. As you proceed through this building, platforms along the walls will raise, ambushing you with imps, so be cautious. Continue through the door at the end of this building (A) where you will find an arachnotron. Once you're back outside, follow the tan brick building ahead of you around to the left. You will pass a smaller brick building to the south - you can enter it for some items if you wish, but it's not necessary to complete the level. Continue circling the larger building until you reach a door in the west. Enter it and drop down inside. Hang a right, following the path between the two blue doors (be careful of the Mancubus hiding in the alcove). At the next split in the path, turn right, snipe at the arachnotron on the ledge, taking cover if you need to, and pick up the blue keycard (B) beneath it.
From the blue key, turn around and head in the opposite to this hallway's end. At the end of this corridor, you will see a small square structure just ahead. Enter the door on the side, kill the chaingunner, and pick up the yellow key (C) he was guarding. Now return to the blue doors, killing the mancubi on the way. The eastern door contains little of value, so entering it is completely up to you. Ultimately, open the western blue door and run through the hall before the floor lowers to get the bulk cell at its end. Once the floor has sunken, drop down and go through the teleporter (D), which leads back outside.
Enter the brown building north of the one you were just in, and proceed through the second, yellow-locked door (E). The doorway leading back out of the yellow-door room requires the red key; however, there is no red key, so you will be unable to leave the next room. It contains nine automatically raising-and-lowering lift platforms, each containing a cacodemon that must be killed (some will see you but others do not know you have arrived yet). Five of these alcoves have switches that you must flip to open the level exit (F [From left to right; the first, fourth, fifth, seventh, and ninth]). After all five switches have been activated, a small passage in the northeast of this room will open, letting you access the exit teleport (G).
In the building that leads to the exit, also filled with cacodemons, the door that you go through to enter the big area of the building is yellow coded from the outside and red coded from the inside, but there is no red key in this map. The red key was present when Doom II was first released and was later removed from the map because it has been decided that it was not necessary in order to complete the level. So forth, the only way to leave the very last room before the exit is by simply exiting the level.
Enter the southern building (H) and go up the stairs to the teleport, but do not go through yet. Instead, turn around and run onto the crate with the berserk pack on top. From here, run onto the platform with the switch, press it, and walk onto the lift that lowered. Ride it back up, and you'll be able to reach the high set of crates where the chaingun rests.
Now take the teleporter mentioned in secret #1. Press the switch on the other side and ride the platform up and pick up the soul sphere here (I). You are now inside the main building.
At the center of the main building, go through the southeastern blue door and follow the ceiling light's path across the brown sludge pool. Flip the switch at the end to open an area northwest of the central pillar in the main part of this building (J).
At the center of the main building, go through the southwestern blue door and take the teleporter inside the next area. After you arrive on the other side, instead of jumping down, turn around and drop into the hole to grab some bullets.
There is a certain area on the outer rim of the building (where an arachnotron is located) where an invisible wall blockes any projectiles that hit it (rockets included) and will cause the player to be seriously injured or killed if shooting at the arachnotron. This can be avoided by shooting it at an angle.
Not a bug per se, but there is a red-coded door (on one side) in the level even though there is no red key for it.