Press the switch to lower the walls. Kill 2, 4 or 7 Mancubi you see and more walls will lower, revealing 4, 8 or 12 Arachnotrons (A). Kill them all and you will be able to end the level by pressing the eye switch in the middle of the map (B). Use the blur spheres to avoid the monsters.
Before you exit, hit the switches in this room and the platforms the Mancubi were on will lower, revealing health and weapons. Also, get the ammo and health items in the areas where the Arachnotrons came from. Now go back to the center of the map and exit.
The northwest corner of the map has a secret alcove, housing a BFG9000 and an energy cell pack, along with another exit switch. In single-player or coop, it is inaccessible; it is only opened when a deathmatch player spawns in that location. This is usually discovered when a player uses the iddt or idclipcheat codes. If a player enters the region through the noclip cheat, the cheat can be disabled safely because the wall can be opened from inside. Note: this sector is the same one as the "secret" sector surrounding the open exit, due to the fact that sectors in Doom do not need to be contiguous.
A possible bug or warning that occurs is that if the last 2 arachnotrons die at the same time, it may cause the stair step surrounding the exit in the center to rise twice, making the exit in the center become seemingly inaccessible, but if this happens, you can still access the exit by running across from the lowered northwest or northeast platforms.
Not really a bug, but more of a warning. If this level is played on Nightmare difficulty and/or with the -respawn parameter set, the arachnotrons that endlessly respawn may cause the stair step that surrounds the center exit to continuously rise each time all of them die, making the exit in the center inaccessible.
The alcove in the northwest corner contains a BFG9000, a bulk cell and an additional exit switch (as the central exit is usually not reachable when using -nomonsters on deathmatch mode). However, this alcove is accessible only in deathmatch mode, after a player spawns inside and opens the door.