To access the keys and the exit in PC Final Doom Plutonia
Experiment level 04, Caged:

You begin this level in a small room at the south side of
the map.  In this room are 4 teleporters.  In the northwest
corner is a switch.  Activate the switch and step onto any
teleporter.

You'll appear in the middle of a small octagonal chamber.
Step up to the north wall of this chamber and Activate the
face-like switch there.  The floor at your feet will drop
down away from the surrounding walls, lowering you onto a
large, square platform near the middle of the map.
Three raised walkways lead away from this platform.  Walk
across the north walkway until you come to a plus-shaped
4-way intersection.
Turn left and follow the west walkway through a doorway,
and continue west until you get to the last room on the
west side of the map.

Turn left and go through a doorway in the south wall, and
follow the curving stairway down and around to a platform.
Look for a switch nearby at the edge of the platform and
Activate it; then go back up the curving stairway.
At the top of the stairs turn to face east, and leave this
room through the open doorway in the east wall.  Ahead of
you you'll see a switch high up on a wall, just to the left
of the next doorway ahead of you.

Activate this switch, and the wall section to the left of
the switch will rise up, revealing a new walkway.
Carefully step onto this new walkway, and follow it as it
curves around to the east.  Keep walking east along this
walkway and you'll see the yellow key ahead.  Grab it.
Turn to face south, and you may see a red teleport pad down
in the brown slime below you.  Jump down into the slime and
Run to step on that teleport pad.

You will appear just outside of a large room on the north
side of the map, and your back will be to the door.  Turn
around to face north and open that door.
Quickly step into the room and Run north across the small
square platforms in the green slime to avoid being burned.
At the north end of the room, Activate the switch in the
middle of the wall, and 2 doorways will open up on either
side of the switch.  Both doorways are connected to each
other by a short curving passage.

Step through one of the doorways, and look for a blue skull
switch in the middle of the passage, and Activate it.  (The
switch performs 2 functions, one of which is to re-open the
yellow-keyed door you came through a moment ago.)
Step back out of the passage and Run south across the small
square platforms in the slime, and leave this room.

Head south to the 4-way intersection of walkways.  Continue
south to the large square platform, and proceed south down
some steps, and look for an open doorway on your left.
A narrow, red-lit path leads east.  Follow this path to a
switch you'll see ahead of you in a small recessed spot in
the east wall.

Activate this switch and it lowers, revealing a teleporter
in front of you; Step on. You'll appear on a raised ledge
on the west side of the map, and you'll be facing south.
Walk south along this ledge, and you'll notice a stairway
next to you on your left.  Go up these steps and Activate
the switch you'll find at the top.

Go back down the steps, and grab the radition suit you'll
see nearby.
As you pick up the suit, turn to face southwest and jump
down into the brown slime below.

To the southwest is a small, open, cave-like area with a
metal floor.  Step into this area and grab the blue key
you'll find here.
When you pick up the blue key, turn around to face east,
and walk back into the slime.
Just ahead of you to the east you'll find a teleport pad in
the slime; step on.

You'll appear on another teleport pad in another slime pool
in the northeast quadrant of the map.  Step off of this pad
and then step back on again.
You will again appear just outside of the large room on the
north side of the map, and your back will be to the door.
This time, walk forward (south) until you come to the 4-way
intersection.  At the intersection turn left and walk east
down the walkway until you come to a blue-keyed door.
Open the door, step through and turn left to face north.

Move a few paces north onto the walkway ahead.  On your
right (east) you'll notice a pool of brown slime below, and
a narrow curving stairway that rises up out of the slime.
Jump northeast down into that slime, and quickly Run to get
to the stairway.
Go up these stairs, and Activate the switch you'll find at
the top.

Once you've activated the switch, turn to face west, and
just ahead and below you'll see the walkway you were on a
moment ago.
Take a step back so that you can get a running start, then
Run to jump down onto the walkway.  (Remember that "Run" in
PC/Mac Doom games is typically enabled by holding down the
Shift key.)
If you don't make it onto the walkway, simply run back to
the stairs you were on a moment ago and try again.  (If you
got onto the walkway but continued running off the opposite
edge, simply Run west to the teleporter pad in the brown
slime ahead of you, and step on.)

Go back to the 4-way intersection, and head south to the
large square platform.  Continue south down some steps, and
walk past the open doorway on your left.  Continue south to
the newly opened door ahead of you, and step through into a
small chamber.
Activate the red switch on the south wall of the chamber,
and the wall panels to your left and right will temporarily
open up.

Step through either opening, and follow the curving stairs
down to a small, oblong room where you'll find a pair of
teleport pads.
(Note that the teleport pads are not identical.  The white
one in the northeast corner leads to the exit chamber; the
red one in the northwest corner leads to the large room on
the north side of the map.)

Step onto the white teleport pad, and you'll appear in a
small caged area on the east side of the map.  Behind you
is the Exit switch.  Turn around and Activate it to exit
the map.

PC Doom players can access a demo that illustrates the path
described above by clicking here.