How to find the secret exit in PC Final Doom Plutonia level
15, The Twilight, which leads to level 31, Cyberden:

The secret-exit portal is in a high chamber that's located
near the south edge of the west side of the map.  Below is
one basic path for reaching that portal.

You'll first appear on this level in a small room located
northwest of the map's center, and you'll be facing south
towards a teleporter pad.  Step onto the pad and you'll
materialize in another small room near the south end of the
map.  Activate the switch you'll find here, and an elevator
beneath your feet will lower you.  At the bottom, step out
onto the south end of a long pathway that leads north.

Head north along this pathway until you come to a low 4-way
intersection.  Turn right and follow the path east to a
door, and continue through.  Run to jump across the small
gap ahead, and pick up the yellow key.

Turn around and head back to the 4-way intersection, and
continue forward/west until you get to a switch at the far
end of the path.  Activate this switch and turn left to see
that a narrow bridge has risen up.  Head south down this
bridge and activate the switch you'll find there.  It will
rise up to reveal a short tunnel.  Activate the switch you
find at the end of that tunnel, then go back out, and head
east, back to the 4-way intersection again.

At the 4-way intersection head north onto the wide platform
ahead.  Continue north up the short flight of steps, and at
the top, look ahead a wall section at your left, which is
made of brown metal plates, and marked near the bottom with
yellow symbols.

Activate this wall section like a switch, and it will drop
down to reveal a teleporter at the top of a pedestal.  Run
onto this teleporter pad, and you'll appear in a small room
just south of the secret exit.  Open the door and step onto
the secret-exit portal to be taken to level 31, Cyberden.

PC Plutonia Experiment players can click here to access a
demo that shows how to find the secret exit on this level.