Access the northwest building in Doom II level The Citadel:

From the central courtyard in the middle of the map, head
south and go through the big wooden door.
Turn right and you'll see a brown panel.  Go through that
and you'll enter a room which has a door to your left, a
door to your right (this is the one you'll want), and a
switch in the middle of the room.  Hit the switch and go
through that right-hand door.
As soon as you cross the threshold of that door, continue
straight ahead until you see an opening or window on your
left (which leads outside).
Activate the wall panel located just to the left of that
opening, and a small chamber is revealed.  Step inside and
Activate the back wall like a switch, and a small lift will
drop down.  Step on and ride up.
Walk forward a few paces and turn a little to the right,
and you'll be looking down a short corridor that leads to
the outside.
Give yourself a little room to build up speed, and then Run
straight out through that opening.  The goal is to jump
over a low wall which is outside and below you, so that you
can access the large building at the northwest corner of
the map.  (If you don't make it over the wall below, simply
go back and try again.)

PC Doom II players can click here to access a demo showing
how to find all keys and the exit on this level.