Access the keys and the exit in Doom II level The Citadel:

(Note: To view a video demonstration of acquiring the keys
on this level, click here.)

The exit chamber is in a structure located in the middle of
the north edge of the Automap.  To access the exit, you'll
need the red key, and either one of the other two keys.

For this walkthrough, the central courtyard in the middle
of the Automap will be the universal starting point.  The
keys can be acquired as follows:

RED KEY
-------
From the central courtyard, 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, turn
immediately right.  You'll see a wooden panel a few steps
ahead and to your left.
Go through that, and you'll materialize at the top of a
pillar in a small enclosed area.  You'll see a door high up
on a wall, and two window areas: one to the right of the
door, and one to the left.
Use your pistol (or shotgun if you have no bullets), and
shoot the left-hand window.  This will cause a platform to
rise up just under that doorway.
From the pillar you're standing on, jump onto that platform
under the door.  (If you miss and fall to the floor, find a
button on the side of the platform you raised.  That button
will drop the central pillar, and you can get back on and
ride it up again.)
Now go through the door, follow the path down and you'll be
able to get the red key ahead of you.  Then, jump back down
into the courtyard.

BLUE KEY
--------
From the central courtyard, 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.)
Once you've made the jump, go up the stairs and you'll find
two entrances at the top of the steps.  Use the right-hand
(northern-most) entrance, which leads into a large room.
Inside, go up the small staircase in the southwest corner
of the large room, and you'll step into a small chamber.
Activate the switch you find there, then go back down the
short staircase into the large room below.
Exit the large room though a new opening in the middle of
the north wall, and you'll drop into another, larger room
containing 5 teleporter pads.
Step on the pad in the far-right (northeast) corner of the
room, and you'll be beamed to a chamber in the northeast
corner of the main citadel.
Step southwest out of this chamber, and you'll drop down to
a lower area.  Take another few steps southwest, and you'll
drop down again to a small platform where the blue key is.
Grab the key and step off this platform, to drop back down
into the main courtyard.

YELLOW KEY
----------
From the central courtyard, 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 (this is the one you'll want now), a door to your
right, and a switch in the middle of the room.  Hit the
switch and go through that left-hand door.
Now, you'll have to be quick during the following maneuver:

Turn left and run down the short hallway to a door you'll
see in front of you.  Run to the door but don't open it.
Now turn around and run back out into the room you just
came from.  You'll see that a panel in the wall has opened
to your immediate left.  Run in there before it closes.

Now walk ahead, hugging the left-hand wall... you'll be
walking along a counterclockwise spiral here.
You'll get to what appears to be a dead end, but the wall
in front of you will drop as you appoach it.
Keep walking, staying to the left.  You'll see a window in
front of you that leads back into the room you came from a
minute ago.  Ignore that window for the moment and look
left.  You'll see a very short hallway, and at the end of
it is the yellow key and a switch.  Grab the key; the
switch is optional.
Go back to the window you just passed a moment ago, and
jump down into the room.
The door to your immediate right is the one that you went
in and out of a minute ago.  On the opposite wall is the
door that led to the red key.  Between these two doors is a
third, the one you used to get into this room originally.
Go out through that third door.
Look to the right and you'll see a big wooden door in the
room.  Go through that door to get back outside the castle.

THE EXIT
--------
Walk around the outside of the castle until you get to that
structure near the middle of the northern edge of the map.
Go in, and you'll find a doorway with three vertical bars:
blue, red, and yellow.  Use the keys to raise the bars.
Step on the teleport pad in the center of the room to exit.

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