Get all keys on PSX Final Doom level 18, Nukage Processing:

YELLOW KEY
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A large, irregularly-shaped courtyard dominates most of the
southeastern quadrant of this level.
The yellow key is located in a small box-like structure at
the northern-most end of this courtyard.

BLUE KEY
--------
A large, irregularly-shaped courtyard dominates most of the
southeastern quadrant of this level.
The blue key is located on top of a capsule-shaped pedestal
at the southwestern corner of this courtyard.
To get the key, first look for the 5-sided pedestal in the
extreme southeastern corner of this courtyard.  (On the
Automap, this pedestal appears somewhat like a classic barn
shape, tipped counterclockwise about 45 degrees.)
There is a lift built in to the back of this pedestal (its
southeastern edge).  Activate the lift and it drops down.
Ride the lift up, then turn south.  Walk or Run off of the
pedestal to land on the raised ledge to the south.
Turn right and walk west to a nearby teleport pad.  Step on
and you'll be teleported west along the south edge of the
courtyard.
You'll materialize facing west.  Continue west to the next
teleport pad ahead, and step on.  Again you'll be beamed
further to the west.
Step off the teleport pad you appear on, and walk a couple
more paces west.
Turn right and you'll see the capsule-shaped pedestal where
the blue key rests.
Back up so that your back is against the south wall behind
you, then side-step left or right so that you're facing
directly towards the blue key.
Now Run to jump the gap and land on the pedestal, grabbing
the key.  Remember that Run in PSX Doom games is typically
enabled by holding down the Square button.

RED KEY
-------
In the center of this level is an 8-sided courtyard, which
is divided by a plus-shaped walkway into four wide pools of
green slime.
The red key is located in a large chamber to the east of
that courtyard.
In this chamber is a tall, wide, round vat of green slime.
The key is floating on top of the slime in the middle of
the vat.
To the east of the vat is a raised platform.  You can get
to it via a stairway which is located a few paces to the
northwest of the vat of slime.  Once you're up in the
raised area, you can look west towards the top of the vat.
The vat may be closed off around the top edge.  To open it,
look for a switch near you on the north wall.  Activating
it removes the barrier.
To get the red key, step back a few paces east, away from
the edge of the platform, turn to face the red key in the
middle of the slime, and then Run to jump to the top of the
vat and grab the key.  Remember that Run in PSX Doom games
is typically enabled by holding down the Square button.