Doom II: Hell on Earth/MAP11: 'O' of Destruction!

Shortcuts

 * You can unlock the exit without obtaining the red key. To do so, head to the north of the circle - you should see a window in the north west area that is just next to the outcropping wall.  A switch should be visible from this window.  Jump into the nukage pit, and get as close to the switch as possible.  Press the switch to unblock the exit.