Reach the keys and the exit in Master Levels map GARRISON:

At the beginning of this mission you'll be at the corner of
a raised walkway near the center of the map, and you'll be
facing southeast.

BLUE KEY
========
A few paces to the south of your starting position, you can
see the blue key, located on top of a tall pedestal.
From your starting position, turn away from the blue key
and walk east a few paces, then north to get down into the
small courtyard.  On the west side of this courtyard is a
blue-keyed door; ignore it for now, and head north into the
wide, enclosed chamber beyond.
As you step into this wide chamber, notice the slot in the
wall to your left.  (Through this slot you can look into a
small triangular room, and see a switch and a teleporter.)
Just to the right of this slot is the east wall of a small
stone booth.
Walk around to the back side of this booth, and you'll see
a vertical slab with a face on it, blocking access to the
booth.
Go back to the southeast corner of this wide chamber, and
you'll see a skull-faced switch on the wall next to a small
square lift in the floor.
Activate that switch (you can do this without dropping down
on the lift) and that slab at the back of the stone booth
will rise up; Run to step on the teleporter inside it.
You'll appear on another teleporter pad inside the small
triangular room.
Step off and activate the switch in the south corner, then
step back on the teleporter again.
You'll appear back at your original starting position, at
the corner of a raised walkway near the center of the map.
Head south, then east, to the top of the steps.  Go down to
the smaller courtyard where you can retrieve the blue key.

YELLOW KEY
==========
Once you have the blue key, return to the courtyard at the
north side of the map.  Go west through the blue-keyed door
and head south.  Activate the wide wooden elevator you'll
find in the corner; get on and ride up.
Continue south into a large chamber, and go down into the
water.  Leave this chamber through an opening in its west
wall, then follow the water south until you come to the
bottom of a series of low steps that lead up to the north.
Go up and continue into the next room to the north.
In here is the yellow key on a small pedestal, surrounded
on three sides by a staircase of wide steps.
In order to get the yellow key, you must drop down upon it
from above.  To do this, first step up onto the low wall
(or "railing") which is just to the south of the yellow-key
pedestal.  Although it can be tricky, it is possible to
step diagonally onto this low wall from the slightly higher
platform to the southwest.

Below is a diagram representing a magnified section of the
Automap.  Here, the yellow-key pedestal ("YK") and part of
the surrounding stairway are shown.


|                      |_|          |
|                      |_|----------|
|                      | |  Third   |
|                 .----| |  step    |
|                 | YK | |  down    |
|_________________|____|_|          |
|__________|_|____2____|_|----------|
|           |           |           |
|    Top    |   First   |  Second   |
|           |   step    |  step     |
|           |   down    |  down    /
| 1         |___________|_________/
|          |______
|                 \
|                  \


A player starting at position "1" can walk or run to cross
the low wall at position "2", and drop down to the top of
the yellow-key pedestal ("YK").  Alternatively, you can try
crossing the low wall to the east of pedestal, by starting
from the second step down (southeast corner of the stairs).

When you grab the yellow key, go back up the winding stairs
to the top, then south into the next room.  Continue down
the steps, then head southeast to the nearby corner where a
teleporter sits in the water, and step on.
You'll appear back at your original starting position, at
the corner of a raised walkway near the center of the map.

RED KEY
=======
At the beginning of this mission you'll be at the corner of
a raised walkway near the center of the map, and you'll be
facing southeast.
From there, walk east a few paces, then north to get down
into the small courtyard.  Continue north (passing the
blue-keyed door to the west), and enter the wide, enclosed
chamber beyond.
Go to the southeast corner of this wide chamber, and you'll
see a skull-faced switch on the wall next to a small square
lift in the floor.
Step on the lift and you'll drop down into a stone-floored
chamber.  (Nearby to the southeast is a wooden structure,
behind which is a small pedestal where the red key sits.)
Head east, following the north edge of the map, then turn
to follow the passage south, avoiding the crushers.
Just southwest past the crushers is a wide wooden platform.
Take note of the narrow walkway leading south to the switch
beyond.  This walkway will collapse as you walk across it;
take care not to fall into the slime pit below.
Walk up to the switch and activate it; then turn left and
run along the remaining walkway to jump down to the wooden
flooring at the east edge of the room.
A teleporter is now accessible in the southeast corner of
this room.  Step on, and you'll appear on another teleport
pad atop a small platform in the southeast corner of the
previous room.
Step off this teleporter, then step back on again.  You'll
appear on top of the red-key pedestal.  Simply step off to
pick it up.

THE EXIT
========
Once you have all three keys, return to the room with the
crushers on the east side of the map.  Head south into the
next room with the slime pit.
Go over to the blue-keyed door in the northeast corner of
this room, and step through.
Turn left and head north to the far end of the walkway.  Go
down the steps, then follow the narrow stone walkway south,
then west, to its far end.  Activate the switch you'll find
there.
A narrow, winding stone path will rise up from far below.
One end will emerge next to you.  Step on and follow it to
the sealed exit chamber beyond.  Use the keys to open the
bars that block access to the exit switch.
(If you fall while trying to get to the exit, quickly run
through the slime to reach the teleporter in the southeast
corner, which returns you to the level's start position.)

Done!

PC Doom players can download a demo of this level showing
how to access the keys and the exit, by clicking here.