Access the keys and the exit in level 19, House of Pain:

When you first appear on this level, you start in a small
rectangular room on the north end of the map, and you'll be
facing south, looking down a hallway towards a teleporter
pad at the far end.
Go step on the teleporter, and you'll reappear in a tiny
teleporter booth near the middle of the map.  Step out and
go west, passing across a very short north-south hallway.
You'll be entering the north end of a long passageway that
leads south.
As you walk south, you'll go past two windowed areas on
your right.
Continue south, following the right-hand wall until you
turn east and eventually come to a wide dead-end.  Look for
the blue key in a nearby corner, and grab it.
Turn and go west, following the right-hand wall.  Continue
following the right-hand wall as it bends north, then east
again.
You'll come to a cluster of barrels and a blue-keyed door.
Open the door and step into a short hallway.  Look to your
left for a door, and open it.
Head north up the narrow hallway until you come to another
door, and step through.
Turn left and open the door next to you (in the southwest
corner of the room) and step through.
You'll enter into a fairly large room, and you'll see three
more doors: one ahead of you to the west, and another door
to either side of you.
Go straight ahead to the west-side door and open it.
You'll enter a small chamber with some health vials on the
floor, and another door/panel to the west.
Open that door/panel, and you'll see a small room with a
corrosive floor.  The red key is in here.  Grab it and step
back into the adjacent chamber where the health vials were.
Step back east into the large room with the multiple doors,
and go out through the east-side door.
Look for a door just ahead and to the right, and open it.
Go south down the narrow hallway and through the door at
the end.
Turn left and you'll see a red-keyed door in the east wall.
Open it and step through.  You'll be at the south end of an
oblong chamber.
Turn left and you'll see a door ahead to the north.  (This
is actually one of a pair of doors that are set against
each other, back to back.)
Near the north side of this chamber is a square depression
in the floor against a corner.  Two edges of the square may
be walked across: the west and north edges.
Slowly step into the square depression across its west
edge, and stop inside the square.  This will open the first
of the two doors.
Now, slowly take one step across its north edge, then step
back into the square.  The other door is now open.
Step out of the square across its west edge, then go north
through the pair of open doors.
Follow the winding hallway around until you come to an
outdoor path that leads north to a small open courtyard.
In the northeast corner of the courtyard you'll find a
yellow-keyed door.  Just to the left of the door is a small
staircase that leads up to a platform where the yellow key
sits; grab it.
Open the yellow-keyed door to access the exit switch.