To access all keys and the exit in TNT level 18, Mill:

When you first begin this level, you'll start in a large
square room in the middle of the map.  You'll be on a
raised platform in the center of the room.  There are 4
main doors in the room: one door in the middle of each of
the 4 walls.

Note: This room will be the starting point of reference for
finding each of the keys.

BLUE KEY
--------
From the level's starting point, go out through the
east-side door, and continue east, past a hall on your
right, and past the elevator on your left.
Continue east and look for an opening on your right.  The
opening is at the top of a stairway which leads down into a
huge room that occupies the east side of the map.  Go down
into this room.
The floor of this room is crusted lava, and is not harmful.
The blue key is located at the top of a tall pillar in the
northeast corner of the room.
To the south of this pillar is a short stairway which leads
up to a low ledge that runs along the east and south wall
of the room.
Get on the ledge and follow it around to a doorway on your
left, in the middle of the room's south wall.
Go into the small chamber there and Activate the switch
you'll see on the wall.
Go back out and drop back down into the large main room.
Look in the southeast corner of the room for a large
4-walled structure.  The doors on this structure were
opened when you Activated that switch a moment ago.
Enter this structure, and the doors will close behind you.
Activate the switch inside this structure.  This will cause
a tall, narrow section of the wall behind you to drop down.
(which will take several seconds).
A teleport pad will be accessible in the southeast corner
of the structure once this wall section has lowered into
the floor.
Step on this teleporter and you'll appear at the top of the
tall pillar where the blue key is located.  When you move
in any direction, you'll pick it up automatically.

RED KEY
-------
From the level's starting point, go out through the
west-side door.  This opens onto the middle of a hallway
that runs north-south.
When you step into the hallway, just ahead of you and a
little to the right is a wide opening in the wall, with
some stairs at the bottom.  Walk west up the stairway, and
continue west down some other stairs, into a large room.
If you turn left at the bottom of the stairs, and walk a
few paces south, you'll see the red key on the floor of a
cage in the corner of the room.
The red key is guarded by 2 barriers: a visible grate-like
barrier, and an invisible one behind that.
Look for a door on the north wall of the room you're in;
Open it and Activate the switch you'll see there.  That
switch opens a short, narrow passage to your immediate
right, and simultaneously raises the "grate" barrier that
surrounds the red key.
Stepping into this narrow passage will lower the invisible
barrier around the red key.
(Activating the switch you see at the end of this passage
is not necessary for acquiring the key.  That switch opens
a locked door in the west wall of the larger room.)
Simply go back to the red key and grab it.

YELLOW KEY
----------
Note: You will need the blue key and red key before you can
access the yellow key.
From the level's starting point, go out through the
south-side door.  This opens onto the middle of a hallway
that runs east-west.
Turn left and walk east down the hall a few paces until you
get to an opening your right.
Step through, and you'll see a flashing wall panel ahead of
you to the south, with 4 red markings along it's top half.
This is a locked door that can be opened with the red key.
Go through the door and continue south down a hallway until
you come to a tall chamber with a brown liquid floor.
Note that around the perimeter of the room are tall wooden
columns.
There is a blue skull switch in the middle of the south
wall of the room.  Activate this switch, and all 6 columns
drop down temporarily, then rise again.
Get on the western-most column (off to the far right of the
switch) and ride it up.
Walk west into a blue-carpeted chamber, and continue west
down the hall.  You'll come to an open area on your right.
The yellow key is here on a pillar, surrounded by barriers.
Continue west until you get to the end of the hall, then
look for an open chamber around the corner to your left.
Activate the switch you'll find on the wall, and a nearby
lift will drop down behind you.  Get on, and it will rise
up to a small room.  Activate the switch you find here,
then go back down.
Leave this chamber and head north down the hallway, past a
blue-rimmed door on your left, to another chamber to the
northeast.
Activate the switch you'll find here, and again a lift will
drop down behind you.  Get on, and when it rises up, look
for another red switch ahead of you and Activate it.
Ride the lift back down, and then head south, back down the
hall until you get to the blue-rimmed door on your right.
Go through the door into a silver room.  Cross this room to
a door on the opposite wall, and go through.
Turn left and head south to a lift you'll see at the end of
the hall.
To the left of that lift is a switch, behind a couple of
narrow vertical barriers.  Activate the lift and it drops
down momentarily.  When the lift is down, the barriers in
front of the switch also drop.
Activate this switch, then leave the lift and walk a few
paces north, back down the hall.
Look for an open doorway on your left, and go through.
Ahead you'll see a switch, recessed into the far wall of a
blue room.
Approach the switch, and get on the low metallic platform
in front of it.
Turn your back to the switch, and face northeast.  You'll
see a pair of lion-faced panels recessed into the corner of
the room to the left of the doorway.
Take your pistol (or shotgun or chaingun), and shoot a
bullet into those left-hand panels.  This will cause the
platform you're on to rise up, allowing you access to the
red switch in the wall.
Activate that switch, then leave this room and go back into
the nearby silver room you walked through a minute ago.
In the middle of this room is a 4-sided column, with a
switch on each side.  These are the controls that lower the
yellow key and the barriers surrounding it.
Activate all four switches, then head east out of the room.
To get back to the yellow key, take a right out of the
doorway, then your first left.  Continue east down the
blue-carpeted hallway, and you'll find the yellow key on
the floor to your left.

THE EXIT
--------
Note: You will need the yellow key before you can access
the exit.
From the level's starting point, go out through the
east-side door, and continue east, past a hall on your
right, and go to the low elevator/lift on your left.
Activate the switch to the right of the lift, step on and
ride up.
Walk west a pace or two, then take your first right.  Walk
north a few paces and you'll see a Computer Area Map item
on your left.
Continue north until you get to a square, brown metal door
with 4 yellow markings at the top.  To the left and right
of the door are metal grates that allow you to see into the
next room.
Go through this door to enter into the southeast corner of
a huge, irregularly shaped room.  Continue north and look
to your left for a low-walled "track" running north-south,
bisecting the north half of the room.
Cross this track to get to the northwest corner of the
room.  Look here for a recessed square panel on the wall.
Walk up to that panel and punch it, or buzz it once with
your chainsaw.  (You can also activate it by shooting it
with bullets or shotgun pellets.)
Doing this opens a silver door in the southeast corner of
this room.  Go through that door and Activate the switch
you see there.
This will open an exit portal in the middle of the north
wall of the room.
Step on to the exit teleporter and you're done.

To access demos that show how to acquire the keys and open
the exit on this level, click here.