How to access the secret exit on Doom 64 level 4, Holding
Area, which leads to level 29, Outpost Omega:

Go to the blue-keyed door that opens to reveal the regular
exit chamber.
This exit chamber is at one end of a row of four narrow
balconies, and a curving stairway is at the opposite end.
At the far end of each narrow balcony is a switch.
Consider these switches to be labeled as Switch 1 through
4, where Switch 1 is closest to the stairs, and Switch 4 is
closest to the blue-keyed door.


      |  \__/  \__/  \__/  \__/  |
  ____|                          |
 /  __|   __    __    __    __   |
|  |  |  | 1|  | 2|  | 3|  | 4|  |
|  |  |  |  |  |  |  |  |  |  |  |  _____
|  |__|__|  |__|  |__|  |__|  |__|_/     \
 \____|                           |Blue   | Regular
      [___________________-___-__-|door   | exit
   /        '  '   '  '   |      | \_____/
Top end of                |    []|
stairway                  |______| Teleporter pad


Activate the switches in this order: 3, 1, 2, 4.
The switches control the large cylindrical column sections
that are located just out beyond the ends of the balconies.
When the switches are activated in the proper sequence, the
horizontal gaps in the columns will all line up with each
other.
Step on to the teleporter pad located in the room just to
the right of the entrance to the regular-exit chamber.
You'll be beamed to a tiny room, located up in a nearby
wall.  This room looks out along the columns you adjusted a
moment ago.
The columns form a line leading from the small room you
stand in, to another small room located up in the opposite
wall.  That distant room is the secret-exit chamber.
Note: There will be monsters present there.
Back up to get a running start, and then Run (remember your
Run button) across the columns to the secret-exit chamber.