To access the keys and the secret exit in PC TNT/Evilution
level 31, Pharaoh, which leads to level 32, Caribbean:

Note: The original version of the TNT Evilution WAD file
contains an error on this map: The yellow key is missing.
For information on applying a file patch to correct this
problem, access http://classicdoom.com/pctnt31p.htm
For a downloadable demo that shows how to escape this level
without the use of file patches or cheat codes, access
http://classicdoom.com/tntdemos.htm#31b

The following walkthrough is for the complete/corrected
version of this map, where the yellow key is present.

When you first appear on this level, you'll be in a corner
of a small courtyard at the south end of the map.
Go through the small open doorway in the west wall of this
courtyard, and look to the right for a small stone slab
hanging down from the ceiling.  Activate the switch on the
back of this slab, then step back out into the courtyard.

Look left for an opening into the pyramid at the north side
of the courtyard.  Go north through the short tunnel, and
you'll emerge into the south end of a large rectangular
room.  Turn right and enter the small opening in the wall.

Follow the passage down the winding stairs, through a small
room, and down a second set of stairs.
Proceed east into a dimly lit room with a water-covered
floor.  In the southeast corner of this room is a pillar
standing at the center of a low wooden base.  On the back
side of this pillar is a switch.  Activate it, then go back
up the steps, back through the small room, and back up the
other set of steps, and back to the large rectangular room.
Back in the large rectangular room, look for a tall opening
in the middle of the west wall, and go up the large stone
steps to a small chamber at the top.
Step into the teleporter booth in the corner.

You'll materialize at the southeast corner of a simple maze
of dark, twisting tunnels on the west side of the map.
Proceed to the northwest corner of this maze, where you'll
find a well-lit chamber.  You can find the yellow key here.
If the yellow key is not present, you'll need a corrected
version of this map.  For more information, access
http://classicdoom.com/pctnt31p.htm
Once you have the yellow key, proceed back to the southeast
corner of the small maze, and step into the small, dark
teleporter booth there.

You'll materialize back in the small chamber at the top of
the stairs where you were a minute ago.  Go down the steps
and back into the large rectangular room.
Turn left and proceed north.  Step through the wide opening
in the north wall to drop down into a room where two large
gray thrones can be found.
Turn right and open the yellow-keyed barrier you'll see in
the east wall, then activate the switch behind the barrier.
Now activate the red switch on the south wall of the throne
room, and a set of steps will rise up to the opening in the
south wall.
Go up those steps and proceed south, all the way to the far
end of the large rectangular room.  Go south through the
narrow passage to return to the outdoor courtyard where you
first began on this level.
Look to the southeast corner of the courtyard for a newly
revealed teleporter, and step on.

You'll materialize at the south edge of a large room on the
east side of the map.  Step into the room and turn left.
Look for stairs ahead of you on the west side of the room,
and go up to the wide, curving platform at the top.
Go to the southeast corner of the platform and activate the
switch you'll find on the wall there.
Step off the platform to drop back down into the central
part of the room.

Head east, through a wide opening in the wall, to enter
another large room where you'll find six cages atop square,
stone pedestals.
Look for a switch in the northwest corner of this room, and
activate it.  Go east to the far end of this room, where a
pair of side-by-side switches can now be found.
Activate these switches, and a pair of blank stone slabs
will rise up at the northwest and southwest corners of this
room.  Each one of these slabs has an invisible switch that
faces towards the east side of the room.  Step up to each
slab and Activate them, then leave this room to re-enter
the larger room to the west.

Head to the west side of the room, and go up the right-hand
staircase.  Turn right and follow the platform east, to an
opening in the northeast corner.
Step into this opening to drop down into a small chamber.
Look around the corner to your left for a switch on the
wall, and activate it.  Go back to the other side of this
chamber, and activate the new switch on the wall there.
An elevator will lift you up and back to the main platform.
Step southeast off the platform to drop into the central
part of the main room.

Search the north wall for a tall wooden door that should
now be raised halfway up, revealing a new stone passage
leading north.  Step into that passage, turn left, and
follow the long narrow tunnel west, until you come to a
torch at an intersection.  Turn left and follow the tunnel
south to a set of steps, and go up.  You may wish to save
your game here.

Follow the passage around until you come to a platform that
overlooks the north end of the large rectangular room you
were in a while ago.  Don't step off this platform yet.
If you look west from the edge of the platform you're on,
you may see the blue key, sitting on a short stone pedestal
in the northwest part of the room below.  To the right
(north) of that low pedestal is a taller structure.  The
lower part of that structure has 3 stone sidings that will
temporarily slide down into the floor when you Activate a
switch located in a small recess to your right.
Activate that switch and quickly jump down off the platform
and run over to that structure in the northwest corner of
the room.  Step onto one of the lowered stone sidings, and
ride it up to a platform at the top.

Jump down to the low pedestal where the blue key sits, and
grab it.  Now head south, to the far end of the room.
Again go through the narrow passage to get back to the
courtyard where you first appeared on this level.
Go to the southwest corner of the courtyard and find the
blue-keyed barriers.  Open them to reveal a new teleporter,
and step on.

You'll materialize at the north edge of a large open area
at the northeast corner of the map.  Near the middle of
this open area, look for a stone and wood structure, and
enter it through the small door on its south side.
Activate the switch you see inside, then exit the structure
and head southeast, where the red key sits atop a round
stone pedestal.  Climb the spiral stairs to grab the key,
and a low barrier wall will rise up to the north, blocking
access to the teleporter you emerged from a minute ago.

From the red-key pedestal, head north a few paces, and look
to the right for a switch on a small slab projecting out of
the ground.
Activate this switch, and the nearby east and south walls
will begin to sink down, revealing other sections of this
now-larger open area.
While standing at the switch you just activated, turn right
and go forward, south, past the red-key pedestal on your
right, and continue south to a gap between some rock walls
ahead.  Follow the short passage/crevasse around to the
right, and activate the switch you'll find there.
Step back out of this passage and turn left.  Follow the
wall to your left, moving west and then south, around the
corner to another switch, and activate it.  This lowers the
barrier that was blocking access to the teleporter.
Go back into the main open area, and head north, back to
the teleporter, and step on.

You'll materialize back in the courtyard where you first
appeared on this level.
Go north into the pyramid, and continue north through the
large rectangular room, through the opening at the far end,
and down the steps into the throne room.
Turn left, and open the twin red-keyed barriers in the west
wall.  This grants access to a switch that will temporarily
open the center section of the throne on far side (east
side) of the room.
Activate that switch and run to the throne, then activate
the new switch that's revealed there.  The door in the
north wall of the throne room will open temporarily; step
through before it closes.

You'll be at the south end of a long hallway that stretches
north.  Go to the far end, and up the steps to the circular
platform there.  Activate the switch at the north end of
the platform, then get down and step back into the hallway.
In the hallway, turn left and step into a newly-revealed
room to the east.  At the south end of this long room is a
switch on the wall; Activate it.
Go west, across the hallway, and into another, larger room
to the west.
Continue west, up the staircase, across the grass, and onto
a large wooden pier.

Drop into the water and look for an exit.  To the southwest
is the exit to map 16, Deepest Reaches; To the north you'll
find a blank stone wall between some stone outcroppings.
Activate that wall to open a rectangular room filled with
health potions and armor helmets.  Go north to the back
wall of this room and Activate it like a switch.  The wall
opens to reveal the secret-exit switch.  Activate it to go
to level 32, Caribbean.

PC TNT players can click here to access a demo that shows
how to access all keys and the regular exit on this level.
(The secret exit is located to the northeast of the regular
exit.)