To get the keys in PSX Doom level 30, Threshold of Pain:

There are 2 keys on this level; blue and red.  To get the
blue key, you first need the red one.

RED KEY
=======
From the level's starting point, walk down the stairs and
proceed west to a door in the corner of the room ahead.
Go through the door into the next room, and go to the far
end.  Leave this room by going west down the hallway.
Continue west, going past the branching passages to your
left and right.
The hallway eventually takes a right-angle turn to the
north.  Continue north, up the stairs, to the far end of
the hallway.
You'll come to a passage on your left that leads southwest,
then south, to a wide door that automatically opens in the
west wall at the top of a dark stairway.  If the door
closes before you get to it, you can open it again by hand.

Go through this door, and drop down into the courtyard
ahead of you.
This courtyard has several rectangular platforms arrayed
along the ground.
At the north end of the courtyard is a passage that leads
west.
Follow that passage as far as it goes, until you come to a
small open area to the west.
The red skull key sits on the ground in the west corner of
this area.

BLUE KEY
========
When you pick up the red key, head east, back down the
passage, to the large courtyard where the rectangular
platforms are.
To proceed to where you can get the blue key, you'll need
to get up onto these platforms.

You can get onto the southeastern platform by running down
off of the stairs on the south end of the courtyard.
(Remember that "Run" in PSX Doom is typically enabled by
holding down the Square button.)
Alternatively, you can go to the southeast corner of the
courtyard, where you'll find a wide rectangular hole that
you can drop down into.  This leads to the hallway that
runs north, then southwest to the wide door at the top of a
dark staircase.  You can go through this door to drop down
onto the platform at the east side of the courtyard.

Once you're on one of the platforms, Run across them to the
small square platform at the base of the west wall of the
courtyard.
The wall panel behind this platform can be opened like any
normal door.  Go through this panel, and proceed west, to a
red-keyed door ahead of you.
Go through this door, up the short stairway to the edge of
the platform you're on, and drop down off the platform.
Make your way southwest, to a large open area.  Here, a
wide brown walkway runs across a corrosive red pool.
Go to the southwest corner of this open area, where you'll
find a skull switch at the top of a short flight of steps.

Activate this switch, and a wide doorway opens nearby, in
the south wall of this open area.  The door only stays open
for a few seconds, then closes again.
Run across the red corrosive to go through the door, to a
small platform beyond.  Drop down off the platform into the
long, wide hallway that runs east to west.

Go to east, up the stairs, to the far end of the hallway.
As you approach the east end of the hallway, the west wall
opens at the opposite end of the hall.
There will now be stairs visible at the west end of the
hall.  Go up the stairs to find a switch in the middle of
the west wall.
Activate the switch, and a doorway opens up in the south
wall, at the top of the stairs on the east end of the hall.
Step through this doorway to access the blue skull key.

When you pick up the blue key, you'll be standing on a tall
platform at the end of a hallway that leads east, back into
the rest of the level.