From the start, head straight east and go through one of the two doors. Grab the yellow key (grabbing the berserk pack will release a group demons), and exit the room through the way you came in. Head north in the main corridor and then east to the yellow door. Pass through this area by heading west until you reach a U-turn. Run down this corridor to the end and turn the corner to find the exit switch.
Other points of interest
From the room with the yellow key, you can run outside via the southern window. Run around the outside area to the north, all of the way around the building, and a platform will lower giving you access to a partial invisibility. (Sector 50)
Enter the southernmost slime pit and enter the corridor to the far south. There will be a platform here holding green armor. (Sector 2)