In order to get the red key in the northwest building use the green slime wall to raise the floor.
Other points of interest
There is a BFG9000, the first officially obtainable one (aside from deathmatch) in a small room in the west. The room is blocked by three doors, each is opened by a lion switch. One switch at the eastern side of the central building, one is in the northwestern building, and one is at the eastern building. There is also a teleporter that brings you from the southern building to the switch in the center of the map, providing you a little shortcut to reactivate the switches in case you escaped the BFG room when ambushed by remaining lost souls. (The doors will shut behind you when you enter the room, but you can temporarily open these from inside. The doors stay opened only by using the switches.) To get the BFG, there are at least two ways:
You can perform a rocket jump. Shoot at the closed door running backwards. This lifts you sufficiently to grab the BFG.
You can straferun from outside the room through the opened doors towards the plinth the BFG rests on to grab it.
Ride down the lift west from the map start and ascend the stairs to the south. You end up in a room with many pipes on the walls. One set of pipes in the east is arranged differently from the rest. Press on this portion of the wall to find a compartment (sector 248) containing a blur artifact.
In the northern wall in the southeast of the cyberdemon's cave, there is a stone passageway (sector 195) leading into a large stone room with a pain elemental and three imps. Flip the skull switch in the southeast to unveil a compartment with a box of bullets, an armor bonus (only on Ultra-Violence or higher) and a teleporter. The teleporter leads to another compartment; open the wall directly in front of you to access a normally caged-off area at the entrance of the cave containing a chaingunner, two imps, a megasphere and an invulnerability. Go back into the teleporter you came from to be taken back outside, on the lowermost step of the stairs surrounding the circular pool in the northeast of the cave.
In the building accessible beyond the red key door, there is a giant section of wall in the southern hallway covered with electronics. Open this wall to reveal a secret room (sector 181) with three troopers, a sergeant, a backpack and a box of rockets.
A shotgun guy (thing 130) is placed inside the zero-length sector of the cliffs located in northern part of the map, so it will be impossible to get 100% kills.
An armor bonus (thing 264) is not collectable: The path (sector 216) between wall and teleporter (sector 219, tag 18) is to narrow to walk over without being teleported. There is a teleport destination (thing 34) inside the teleporter so that in theory one could teleport to there and walk to the armor bonus, but since the corresponding linedef is missing, this is impossible.