How to access the keys and the exit in BLACKTWR.WAD:

At the start of the mission, you'll appear at the western
side of the map, in a walled-in outdoor courtyard.  You'll
be facing east towards a large black tower.  To the
northeast of your starting position, about halfway along
the north wall, is a stairway which dips below ground,
leading to the exit chamber, which is guarded by locked
doors and barriers.

Here's how to get the keys to open the exit:

YELLOW KEY
----------
From the western courtyard, proceed east to the black tower
and enter it through the demon-faced door on the tower's
west side.
This tower is basically a tall, hollow, 8-sided tube (or
"cylinder"), and there are four doors at the base: one each
on the north, south, east and west sides of the tower.  You
just came in through the west door.
Turn left and go out of the tower through its north-side
door.  This door leads out to another outdoor courtyard
surrounded by low walls.  Turn northeast, and look for an
opening in the wall in northeast corner of the courtyard.
Enter the opening and follow the short tunnel to a skull
switch on the left-hand wall at the back of the tunnel.
Activate it, and go back to the tower.
Enter the tower through its north-side door, and once
inside, exit the tower again through its east-side door.
Again this door leads out to a courtyard.
In the middle of the courtyard wall ahead of you to the
east is a switch.  Activate it and go back and re-enter the
tower.
As you go back into the tower through its eastern door,
look left (southwest) for a skull-switch sticking out of
the floor on a flattened vertical post.  Activate this
switch, and a spiral staircase will emerge, running up and
along the curved inner wall of the tower.
The base of the stairs begins at the eastern door.  Go up
the stairs, and follow them all the way to the top.  At the
top, look to your right for a catwalk that starts near the
top of the stairs and runs southwest to the top of the
huge, pillar-like structure that occupies the center of the
tower.  Start down the catwalk.
Ahead of you is a teleporter in the center of the platform
that the catwalk leads to.  Step into the teleport area and
you will appear inside a chamber within small structure,
facing south towards a door.  This structure is at the
center of a closed-in 8-sided room, or octagon.

This room is one of five separated, individual octagonal
areas, which are clustered near each other on the east side
of the map.  This first octagonal area is referred to as
"Octagon 1".

Step out of the chamber and turn right, walking clockwise
along the outside of the central structure, until you come
to a chainsaw on your right.  Grab the chainsaw and
activate the face-like switch you'll see on the wall behind
it.  Now walk around to the opposite side (east side) of
this structure, and look for another face-like switch on
the wall, and activate that.  To the left of this
face-switch, some bars will rise up, allowing access to a
skull switch, high up on the east wall of the structure.
Activate this skull switch to open a "cage" at the north
side of the structure.  Go around to the north side, and
note that a single, central bar of the cage is raised,
allowing access to a lion's-head switch on the back wall of
the cage.

Note that as you step up to this switch, this bar becomes a
crushing device, like a pile-driver, and will repeatedly
rise up and push down again in a cycle.  Be careful not to
get caught under it.

Activate the lion switch, and the wall section that this
switch is on will drop, revealing a skull switch behind.
When you activate this skull switch, the outer northwest
corner of the central structure drops down temporarily,
allowing access to a teleporter booth, just outside the
cage and to the right of the skull switch.
Activate the switch, then run backwards, out of the cage,
then run for the nearby northwest corner of the structure.
When you step into the teleport area, you'll re-appear in
"Octagon 2", another octagonal area, which is divided up
into many rooms and chambers.  You'll be facing west, down
a short hallway.  Behind you is a secret door.  Around you
is a corrosive floor.
Go west down the short hallway.  It leads out to a passage
that runs all the way around the perimeter of Octagon 2.
Turn right and follow the passage clockwise around to the
east side of the octagon.  Look for a single brown door
with a face on it, to your right.  Go through the door and
you'll enter into a rectangular room with 4 red pedestals
rising from the floor, and 2 more brown doors on the north
wall.
In front of each door is an explosive barrel.  Destroy the
one in front of the farthest door (in the northwest corner
of the room), and then go through that door.  Turn left and
head west into a smaller, narrow room with a teleporter at
the far end.
Step into the teleporter, and you'll appear on the west
side of "Octagon 3".
In the center of this octagon is a square pillar that
stretches floor to ceiling.  On its south face is a skull
switch.  As you step up to this switch, four walls will
rise around you, boxing you into the middle of the room.
Activate the skull switch.
To drop the walls that now contain you, look for a switch
in the southwest corner, where two of the four walls meet,
and activate it, dropping the walls back into the ground.
Now head for the yellow key on the north side of the
octagon.  (The skull switch on the central pillar dropped a
pedestal that the yellow key was resting on.)

BLUE KEY
--------
When you grab the yellow key near the north wall of Octagon
3, turn west and look for a green torch close by, in the
northwest corner of the room.  To the immediate left of
this torch is a slightly discolored wall texture.  This is
a secret door.  Open it, and step into the triangular room
beyond.  Turn left and walk to the far-southwest point of
the room.  As you approach, a wall panel opens, to reveal a
small, dark, squarish chamber to the south.
An explosive barrel blocks entry to this chamber.  Step
back and destroy the barrel from a safe distance to gain
access to the chamber.  Step inside and look for a switch
on the south wall, and activate it.  Now go back out to the
main room, and back to the tall pillar at the center of the
octagon.
The north side of the pillar has opened to reveal a
teleport booth.  Step in.  You'll appear in "Octagon 4",
facing north.
Centered along the north wall of this octagon is a box-like
structure.  Walk up to the west side of this box, and open
the hidden door there.  In the northeast corner of this box
is a teleporter pad.  Step on.
You'll appear at the center of "Octagon 5", inside a box.
This box is at the center of a 5-box cluster, arranged in a
plus-shaped pattern: There's one box each to the north,
south, east and west of the box you're in.  At your feet is
an Invulnerability Artifact.  You won't be able to move
without picking it up.
The wall you're facing (to the north) will drop if you
activate it like a switch or a door.  Open this wall and
step out.  Turn left and go to the west box.
Move to the south face of the west box and open it.  On the
inside of the box, on the far (north) wall, is a switch,
disguised as a body, hanging upside-down behind some bars.
Step up to this switch (across a corrosive floor) and
activate it.
(Note that this box, and all the others except the center
one, have corrosive floors.)
Now head to the northern box.  On the western face of the
north box is an opening.  Step in, and on the back wall
(east wall) is another switch.  Activate it and head for
the east-side box.
The northern face of the east box has lowered to reveal
some armor.  When you step up to grab the armor, the south
box opens.
When you approach the south box, you'll see that its north
face is open, allowing access to the blue key, which sits
on the corrosive floor of this box.

RED KEY
-------
There is a skull switch on the back wall of the south box
(in Octagon 5) where you got the blue key.  When you grab
the blue key, activate this switch.  Now run for the center
box in the area.  (The switch you just activated opens the
northern face of the center box.)
Run into the center box, and on to the teleporter inside.
You'll re-appear in "Octagon 4" (see "YELLOW KEY" and "BLUE
KEY" for information on the "Octagon rooms").
You'll be in a small chamber at the north side of this
octagon.  There is a skull switch in the middle of the
south wall of the octagon.  Activate this switch to reveal
a teleporter booth in the center of the room.  (When the
skull switch is activated, the north face of the room's
central pillar drops down, allowing access to the
teleporter.)
You'll appear in a teleporter booth at the center of
"Octagon 3".  Step out and note that you're surrounded by a
four-walled box.  Walk to the southwest corner of this box,
and look for a face-like switch.  It will be just visible
behind a pillar.  Although the pillar blocks you from
standing in front of the switch, you can still Activate the
switch from an angle.  Doing so will drop the walls that
surround you.  Walk to the middle of the north wall of the
octagon, and turn left (west).  Look for a nearby green
torch.  To the left of the torch is an opening to a
triangular room.  Enter this room and look for a small
square chamber to the southwest.  Enter this chamber and
look for the red key on your right.  (There is also a
Computer Area Map item here.)

THE EXIT
--------
To exit the level, you will need all three keys.  When you
grab the red key, head to the middle of the west wall of
the octagon ("Octagon 3", where you found the red key).
Once at the west wall, turn right (north) and look for a
glowing, bluish-red teleporter pit.  Step into the
teleporter and you'll appear at one end of a small, narrow
room on the west side of "Octagon 2".
Walk east out of this room, turn right and go out the brown
door.  You'll enter a rectangular room with 4 red pedestals
on the floor.  Turn left and leave this room through the
brown door on the east side.  You'll be in a passage that
runs all the way around the outer edge of Octagon 2.  Turn
left and follow the passage counterclockwise around to the
west side of the octagon, and look for a short open hallway
on your left, leading to a small room with several bodies
hanging upside-down from the ceiling.  Enter this room, and
head for the back wall where you'll find a teleporter pit.
Step on the teleporter and you'll appear in "Octagon 1", on
top of a raised teleporter at the northwest corner of a
small, central structure.  Step down off this structure and
look for an opening into the structure.  The opening is at
the structure's south side.  Enter the structure and step
into the glowing teleporter pit at its center.  You'll
appear on top of a high platform in the center of the black
tower, facing northeast.
Turn to face southwest, and walk forward off the platform.
When you land on the ground, look for a nearby switch on a
flattened vertical post sticking out of the ground.  This
should be a skull switch with black eyes.  Activate this
switch, and leave the black tower through its east-side
door.
Turn left (north) and walk around the outside of the tower
until you come to its west side.  Walk west into the middle
of the western-most courtyard.  Turn north and walk to the
middle of the north wall of the courtyard.  Look for a
stairway that dips down below ground level, to a passage
running north.
Walk north down the stairs and to the end of the passage,
and open the 2 doors there, one behind the other.  This
allows access to an open, lava-walled, vertical shaft.  The
bottom of the shaft is far below, and its floor is
corrosive.
(Warning: The corrosive will even overcome an Invincibility
cheat code.)
The inner-left (west) wall of the shaft is a barrier-door,
which blocks access to the exit chamber, which is a small
room just to the west of the vertical lava shaft.  The red
key is needed to open this barrier.  Once open, run west
into the exit area, and the mission is complete.

To read about or download a demo of this level, click here.