Behind several of the inactive transporters are secret areas, containing switchs to the lighting in the main room, ammo hallways, a rocket launcher, and an invincibility powerup.
There is a secret door in one of the small pentagonal rooms(specifically, the southwest room in the Map view) that opens into a narrow winding passage that ends at another secret door, past which is a small ledge overlooking the arena. (The ledge is visible in the Map view, just south of the west arena entrance). [Ed: It is also almost impossible to see into that ledge from a good distance in the main arena area re: ambush!]
There's a secret door at the bottom of the inverted J at the bottom of the map. Again, an invincibility shield.
In the right-most room on the map, there's a transporter behind a secret door, in the left-most alcove of the north wall. The transporter gets you on the ledge behind the door on the west side. But then, of course, you knew THAT!
Jeremy P. Condit (afn02635@freenet.ufl.edu) points out that
there is a secret room on the right side of first room one reaches
when going downstairs (across from the MA75 alcove). It contains