Hit the switch and follow the caged hallway crossing an intersection and turning left at the end (A). After eliminating the Cacodemons flip the switch and go back into a new access at the corner (B). After dispatching the Imps get the red key and go back to the hallways. Return to the intersection, go west and enter the red door (C).
Cross the pool of toxic waste and climb the stairs. Watch out for the Baron who will be in a small raising section (D). Get the blue key and retrace your steps back out the red door. Follow the whole walkway until you arrive in a huge room with a curved walkway above a pool of toxic waste (E). Watch for Cacodemons to raise up from there and follow the walkway to the blue door.
In that outdoor area, make your way around the walkway to a platform with a yellow key (F). Retrace your way back through the huge room and back into the caged walkways. At the intersection go back north and around the corner and enter the yellow door near the switch you hit for the red key (G). Jump in the hole to exit.
At the level start, immediately turn around and open the wall behind you to reveal a secret area (H).
Return to the starting room and press the switch. Once the floor lowers, open the vented wall to the right (I) to access another fairly extensive secret area.
The wall behind the blue key opens, revealing a switch (J).
Press the switch in secret #3, and return to the previous, lava-floored room. Two timed lifts have been lowered on the north and south walls (K). Ride the northern lift up into an alcove containing a combat armor vest. One of the walls in this alcove is differently textured from the rest. Open it and follow the tunnel to a soul sphere.
The caged room from official secret #1 (see above) is surrounded by a huge moat of green nukage which has a radiation shielding suit and 2 teleporters: one takes you atop the tallest of the 2 pillars overlooking the end of the room platform (from this, you can run onto the top of the smaller pillar to grab the plasma gun), while the other warps you to an otherwise unaccessible ledge containing an energy cell pack.