How to get the red key on PSX Final Doom level 19, Deepest
Reaches:

Note: In the PlayStation 1 version of Final Doom, a red key
appears on this level, but is not used.  A player may
access all accessible areas and exit this level without
acquiring the red key.

Along the south edge of the map are winding, stone-walled
passages with water-covered floors.
Once you have the yellow key, go to the southwest quadrant
of the map and head down to these watery passages.

Look for a long and narrow winding staircase that leads
down and east, to a room where you'll find a green and red
"eye" sculpture floating just above a candle in the middle
of the floor.

Go to the northeast end of this room where you'll find a
yellow-keyed door.  Go through the door and follow the
stairway leading down to the large rectangular room that
dominates the east half of the map.

Step down to the floor of this large rectangular room, and
head west, where you'll see a brown panel on the wall with
a face-like switch on it.
Activate this switch to open a passageway, and walk through
to the small room beyond.

Activate the switch-panel on the right wall of the room to
open a door on the left.
Step through the door, and as you enter the next room
you'll notice 3 large stone blocks in the walls that have
face-like carvings on them.

Turn right, towards the face on the west wall of the room.
Use your fist (or chainsaw, pistol, shotgun or chaingun) to
strike the face, and it will rise up, revealing a short
passage leading west, to a switch on the wall.  The switch
will temporarily lower a long, bench-like stone platform
located against the south wall of the room that's to your
left and slightly below you.
Activate the switch and run south into the next room to get
on that stone platform before it rises up again.

Once you're on the platform and it rises up, look for an
opening in the south wall where you'll see another switch
at the end of a short passage.
Activate that switch, then turn and step back down off the
platform.

Go a few paces northeast to step up into the square room
where those 3 stone faces were.
Notice that the face that used to be on the south wall of
this room has risen up to reveal a doorway.  Step through
and walk forward a few paces.
Look for an opening on your left (east), and step through
to a potato-shaped room with a switch on the east wall.
Activate this switch, then walk out of this room and turn
right (north) and walk back out to that square room where
the 3 stone faces were.

Step southwest, back down into that room where the stone
bench-like platform is.
Look to your left (east) for a newly-revealed switch on the
wall.  Behind you, on the west side of the room, is a stone
pillar that's partially embedded in the wall.
Activate the switch, and the pillar drops down momentarily.
Get on and ride up.

Once on top of the pillar, the red key will be in front of
you, in a small red niche in the west wall.  Simply step
into the niche to grab the key.