In the northwest room of the central building, press on the northwest section of the west wall, and it will lower, allowing access to combat armor. (sector 208) Note that once you open the way into the building containing the red key, you will be unable to reach this secret.
In the building filled with crates at the southeast of the level, there is a wide crate with a computer map sitting on it. There is a tripwire that causes this crate to act like a lift, located on the edge of the upper level, in front of the skull switch in the northwest of the building. While in front of the switch, leap south off the upper floor, and then run as quickly as you can to this computer map before the crate rises to register this secret. (sector 308)
At the north side of the level, enter the northeast side by either jumping off the rooftop between the north and northeast buildings, or going through the north door in the northern building. Head to the north side of the northeast building to find a megasphere, as well as a lift leading to the upper floor. (sector 217)
In the northwest building at the southwest of the level, there is a chainsaw in front of a demon face; pressing on it lowers both the palte and the floor behind you. The north side of the plate (linedef 523) is also flagged to do this, but is untagged. As a result, pushing it will cause every untagged sector to undergo this effect, trapping you inside the room and preventing completion of the map.