Overlooked or inaccessible secret in PlayStation Doom level
53, Monster Condo:

A secret area that may be easily overlooed is a small room
in the middle of the western edge of the Automap.  In the
original PC map, this secret area automatically closes 30
seconds after the game starts, and cannot be reopened again
from the outside.  In the PlayStation version, this room is
closed from the beginning, but can be opened at any time.

Here's how to get there from the single-player starting
position:

Directly ahead of you in the far distance is a doorway you
must go through in order to get to the secret area.
Between you and this doorway is an oblong pit with acidic
slime at the bottom.  On the eastern side of the pit is the
bottom of a stairway that leads west, curving up and around
to a column near the center of the pit.  Walk up the stairs
until you're at the point of the stairway which is closest
to the northern edge of the pit.  (On the Automap this will
be the midpoint of the stairway's curve.)
Look north and you'll see a cluster of skulls on the floor.
Back up a little to get a running start, then Run to jump
from the stairway to the cluster of skulls.  (Remember that
in PlayStation Doom games, "Run" is typically enabled by
holding the Square button down.)
Now head for the door, which is to the northwest of the
skulls.  When you walk through this door you'll see a
partition ahead of you, with a giant cut-out arrow pointing
left.  Turn left and the lowered walls of the secret room
will be ahead of you, in the left-hand corner.
Look for a small glowing "lamp" texture on the wall in the
corner, and activate it like a switch to reveal the secret.

To view a map of this level which shows all score-affecting
secret areas, click here.