Escape the D-shaped room on the Doom II level The Citadel:

You can use your fist, chainsaw, pistol, shotguns or
chaingun to attack any part of the wall in the southwest
corner of the room.  This will cause a pedestal to rise up
to the door in the northwest corner of the room.

PC Doom II players can access a demo showing how to escape
this room by clicking here.