Next to the gold key door, a brick sticks out. Push it, the caged Quad Damage becomes available. I suggest you wait picking it up, until you go trough the gold door?

Jump in the acid pool and hurry into the small room for some protection. At the other end of the walkway is a submerged tunnel, leading to a goodie. Then press the switch next to the small room, to get out of the acid again.

Shoot all the gargoyles on the walls. The back wall in the second alcove in this corridor will open when you shoot it. Apparently, it has Tim Willit's initials inside.