WALKTHROUGH:
    For anyone who get's completely stuck, here's a basic walkthrough.  The trick to the level is to double back so that you get all the power ups and both keys so you can get the BFG.  Both routes are set up so that you have to get at least one key before going forward, and you can always get back the way you came.  There's a chainsaw and a green armor hidden behind the curtains (two-way walls) in the starting room.  I usually take the left route first (the door you are facing when you start).  There's a super shot-gun and ammo hidden in the left computer terminal, the yellow key is hidden a little more conspicously in the terminal behind the imp.  I think the third switch from the left opens both doors, all the other switches just turn the lights off.  In the right route a easy trick is to blow up the barrels before going into the room.  I wanted a room where the type of player who usually runs ahead with guns blazing would have to stop being trigger happy for a bit and figure out how to attack the Pain elemental without getting blown up in the process.  You can make it through without blowing up the big set of barrles first, you just have to be careful about how far away your are.  There's a megasphere, behind the pain elemental and getting it also turns the lights back on.  Leaving the right route you can see the secret passage to the BFG.  Going to the BFG from the wrong way crashes the ceiling in on you, you can only get it if you've gone through both routes - I wanted players to see a big reward early on, but only get it if they were committed to finding everything in the map.  There's a secret passage behind the curtain to the right in the final hallway that gets you the blue key, and to the left in the final hallway there's a computer map (There was an invisibility sphere there, but I find them pointless and I was just looking for more stuff to hide in places).  Every two way wall in the hallways is labeled with a curtain, and the two way walls in the computer room are labeld with a metal grating that sticks out from the rest of the computer terminals.  Every walk-over that does something has that classic brown step texture across it, but if the lights go off in a room they will always be turned back on once the player leaves the room.  To get to the boss room and eventually exit, you have to open the big front door with the blue key, which actually opens a secret passage behind you where the painting was.  The boss is different for every difficulity level but you shouldn't have a problem unless you run out of ammo, which is easy to do on the hardest difficulties.  The exit is behind the door which looks different from the other three walls, and the door itself is in front of a two-way "wall of fire".  If you go right instead of left, there's another two way wall that activates the goto secret level instead of the regular next level.  That's it.

- Jim Albert, "fishjimboding"