To access the exit in PC Doom II level 14, Inmost Dens:

This level is basically composed of four small buildings
that are connected by (and surrounded by) various covered
bridges and walkways, tunnels and waterways.

When you first appear on this level, you'll be on the west
edge of the map.  You'll be facing east, looking towards
the center of a building.
Go east (around this building) until you come to a short
covered bridge that has a large wooden door at either end.
(These doors will open automatically when approached from
outside the bridge, or they can be opened from inside the
bridge by stepping across a lighted rectangle on the ground
at the center of the bridge.)
Go east across that bridge and turn right.  Proceed around
the corner and look to the left left for a short flight of
steps leading up to a door at the top.
Go through the door and into the center of the room.  Look
at the north wall of this room and you'll see a wide,
irregularly shaped window.
Run north through the window, to jump over to a raised
stone walkway, or pier.  (Remember that "Run" in PC Doom
games is typically enabled by holding down the shift key.)
Continue north until you step into a covered walkway, and
turn left.
Follow the walkway around to the red key, and grab it.
Go back to the pier and head back towards the building you
jumped out of a moment ago.
Run south off the pier, to jump back towards that building.
You'll land on the ground near that irregularly shaped
window.
Turn right and walk west, and go back through the covered
bridge with the wooden doors on each end.
Once across that bridge, turn left and follow the path
around to a barrier of red vertical bars on your left.
To the right of that barrier is a locked booth.  Open this
booth with the red key, step inside and Activate the switch
on its back wall.  This drops the barrier to the left of
the booth, allowing access to another covered bridge that
leads south.
Go south across that bridge, and enter the building ahead
to the south.  Grab the blue key you'll find in there, then
return north, back across the covered bridge.
Turn right and head east again, back across the first
covered bridge.
Again, go around to the right, then up the stairs on your
left and into the middle of the room.  Run north out of the
window to again jump to the raised stone pier.
Go north into the covered walkway, and turn to the right.
Follow the path around to where you'll have to jump/step up
onto a stone block protruding out from the end of another
covered walkway.  (If you fall down into the water, simply
activate the stone block like a switch, and it will drop,
allowing you to get back up to the walkway.)
Follow the path around the corner and you'll come to an
opening on your right.
Head south across the raised stone walkway to another small
building.  Enter through its locked door with the blue key.
Inside and ahead, on the back wall to the south, is an
elaborate door/window.  Through that window (across a small
gap) is the door of the exit chamber.
Run across the gap, open the exit chamber and Activate the
switch on the back wall to exit this level.

If at any time you become lost or trapped in the waterways
around this map, look for nearby elevator/pedestals.  You
can usually Run/jump from these to reach dry land.

PC Doom II players can click here to access a demo that
shows how to access the exit on this map.