How to find the secret exit in PC Final Doom TNT/Evilution
level 15, Dead Zone, which leads to level 31, Pharaoh:

The secret-exit switch is south and a bit east of the map's
center, at the back of a small hidden niche that is located
a few feet above ground level in an exterior wall of the
large, main building.  Below is one basic path for reaching
the secret exit.

You'll first appear on this level in a small chamber at the
south end of the map.  Step up to the north wall here and
Activate it like a switch, causing it to rise up.
Walk up the steps to the large outdoor area, and turn right
to face east.

Walk several paces east and you'll see a metallic elevator/
lift on your left; Activate it and ride up.
At the top you'll see a short flight of steps nearby.  Walk
up them into the small room ahead.  At the far side are two
openings, one leading north, the other leading southwest.
Bear left to go through the southwest opening, and continue
along the narrowing ledge to an opening on your right.
Step in and follow the tunnel to the point just before it
bends to the left.

Search the right-hand/east wall here for a secret panel.
Open it to reveal a tiny chamber, and step in.  Now search
the right-hand/southeast wall of this chamber to open
another secret panel, revealing a teleporter pad.

Step on the teleporter and you'll appear in the small
hidden niche in the exterior wall of the main, central
structure.  You'll be facing the outdoor area on the east
side of the map.  Turn around to find the secret-exit
switch behind you.  Activating it will take you to level
31, Pharaoh.

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