How to get to the keys and the exit in Doom 1 level E3M7,
Limbo / Gate to Limbo:

(Note: To watch a YouTube video of the walkthrough below,
click here.)

The game's Automap screen can be a useful reference tool
while playing.  In the instructions below, "north" refers
to a direction toward the top edge of the map screen,
"east" would be the right side of the screen, and so on.
To view a complete map of this level, click here.

You begin the level at the south end of a tunnel that leads
north to a very large, circular room.  This will be the
starting point for basic instructions on where to pick up
all three keys (blue, then red, then yellow), and then get
to the exit.

BLUE KEY
--------
In the large circular room, there are 4 wide doorways: One
each to the north, south, east and west.  You just entered
the circular room from the south doorway.
Turn left and leave this room through its west-side door.
Simply follow the left-hand wall around a U-turn at the
corner to your left, and continue following the left-hand
wall southeast, then south, until you come to the blue
skull key in the corner.  (It's located in the southeast
corner of this general quadrant of the map.)
Note that there are 2 very small, short walkways that lead
across a small square pool of red acid around the blue key.
Both walkways will sink into the acid when you step on
either one of them.  Run across the walkways (or the acid)
to avoid being badly burned.
Now retrace your steps to back to the large circular room.

RED KEY
-------
When you're back in the large circular room, leave through
its east-side doorway.
You'll enter an area that has 2 blue-keyed doors.  Turn
left and go through the north-side door.
In the room ahead you'll see a low, wide stream of red acid
crossing your path.  At either end of the acid stream (to
your left and right) is a small stairway behind a narrow
partition of brick.
Jump down into the acid and run up either stairway to get
up to the north side of the stream.
Continue north and you'll find a hole in the floor, at the
base of the room's north wall.
Drop through the hole, and you'll land in a small stream of
red acid.  Quickly Run north to the chamber ahead.
You'll find radiation suits here; grab one.
There are paths leading east and west out of this chamber;
Turn left and go out through the west-side door.
Go past the doorway on your left, and continue a few paces
west, hugging the right-hand wall, until you come to a
doorway to another small chamber with some radiation suits.
Step into this chamber, and continue west, out of the
chamber's west-side door.
Stick to the left-hand wall as you continue west, then
north.
Go through the first doorway on your left... and turn to
your left.
When you look west down a short passage, you should see a
teleporter pad on the floor in the acid a few paces ahead,
and another opening on your left.
Turn left to go south through that opening, and follow the
short passage west, until you come to a small chamber on
your left.
You'll find the red key here, and another radiation suit.
Grab the key, and use it to open the red-keyed wall panel
here on the south wall of this chamber.
It opens, revealing a switch.  Activate it.
Walk north out of this chamber, past the passage on your
right, to another passage a couple of steps further to the
north.
Walk east down this passage to the teleporter pad.  (You're
now seeing it from the opposite side from where you saw it
before.)
Step on, and you'll be teleported to a small chamber that's
located just to the northwest of where the blue key was.

YELLOW KEY
----------
Once you have the blue and red keys, you want to get to the
teleporter booth in the far southwest corner of the map:
After you pick up the red key and teleport back to the
small chamber near the blue key, step out of the chamber
and turn right.  Go a few paces south, then west.  Continue
west, following the left-hand wall, until you come to a
pool of red acid.
Run into the acid and continue west and north, to a wide,
round brick column ahead.
Go north around to the back side of the column, and look
for a door there.
Step on to the teleporter that's inside the column, and
you'll be sent to a small narrow chamber in the northwest
quadrant of the map.
Walk to the end of this chamber and look for a switch on
your right, and Activate it.
Walk back to the other end of the chamber and step back on
the teleporter.
You'll appear back inside the brick column where you were a
moment ago.
Step out, and Run south and east to get back to a solid
floor.
Now you'll want to get back to the large circular room near
where you started the level: When you step out of the red
acid, turn left, and follow the left-hand wall north,
around to a wide opening in the wall that leads north.
Step through that opening and immediately turn right.  The
large circular room is ahead to the east.
From within the large circular room, Run out through its
north-side doorway to enter into another large acid-floored
room.
Here you'll see two more wide brick columns: One to the
east, and a slightly more distant one to the northwest.
Run to the back side of the northwest column, and step
through the door to the teleporter inside.
You'll appear on a raised platform in a large room at the
northwest corner of the map.
As you appear here, look on the wall on your left for a
switch, and Activate it.  Then step back on the teleport
pad next to you.
You'll appear back in the brick column where you were a
moment ago.
Step out, and quickly Run a few paces south, then east, to
the other column on the east side of the room.  (Note that
there is another radiation suit in a small cul-de-sac just
to the south of this column, if you need it.)
Step through the door of this column to the teleporter
there.
You'll appear on a different platform in that room in the
northwest corner of the map.
The yellow key will be on the platform with you, a few
paces ahead and to the right.  Grab it and step back on the
teleport pad.
You'll appear back in the brick column where you were a
moment ago.
Step out, and quickly Run south, back to the large circular
room near where you began this level.

THE EXIT
--------
Once you have all three keys, and have Activated all the
switches mentioned above, go to the wide brick column at
the center of the large circular room near where you began
the level.
Step through the door of this column onto the teleport pad
inside.
You'll appear in a rectangular room on the west side of the
map.  Ahead, off to the left, is a yellow-keyed door across
a small bridge.
Open the door to reveal a small chamber.  Look inside to
the right for a switch on the wall, and Activate it.
Now go step back onto the teleporter, and you'll reappear
back in the brick column where you were a moment ago.
Step out of the column, back into the large circular room.
Turn left and Run west across the acid to leave this room
through its west-side door.
As you step into the doorway, follow the right-hand wall in
a west/north/east U-turn around the corner to your right.
Walk a few paces east to the end of the hallway, and you'll
see an acid-floored chamber on your left.
Step north into the chamber, and go around to the back of
the wide brick pillar you'll see there.
Go through the door in the column, to the teleporter pad
inside it.
You'll appear in a small room on the west side of the map.
Ahead of you is a yellow-keyed door.  Go through, and
follow the path north, across a bridge to the exit chamber.
Open it, and simply step on the exit teleporter inside to
finish the level.

To read about and/or download a PC compatible demo of this
level, showing how to access all keys and get to the exit,
click here.