How to find the secret exit on level 29, Twilight Descends:

Once you've gotten the red key and explored the level, take
a look at your Automap.
In the northwest corner of the map is a tunnel that runs
east-west along the north edge of the map.  The longest
part of the tunnel is divided into 4 "segments".  On the
map, the segments are divided by a slight pinching-in of
the tunnel walls.
On the map, look at the second segment in from the left.
On the south wall of this segment (in the southwest corner
of the segment), is a semi-hidden door.  This door leads
south into a convoluted chamber, which is basically divided
into two sections: A west-side section where there are 3
torches recessed into the wall, and an east section with a
small closet-like structure in the north wall.
The back wall of that "closet" is a lift which drops down
for a couple of seconds whenever the middle torch in the
west section of the chamber is activated.
The trick (and it's a hard one) is to activate that middle
torch in the west section, then run to the east section and
get on the lift before it rises.
This may take several tries.
Remember that Run on the PlayStation 1 is typically enabled
by holding down the Square button on the controller.  (Some
people don't know, or forget, that "running" is an option
in Doom games. ;)
If/when you make it onto that lift, it will raise you up to
the west end of a short east-west tunnel.
Simply walk to the end of the tunnel, to where a teleporter
pad will take you to level 57, The Marshes.