To enter the northwest room and get the plasma rifle on
level 5, The Waste Tunnels:

Background: On the west half of the level, along the north
edge of the map, is a small rectangular room, where you can
find a plasma rifle.  Entering this room counts towards the
player's Secret score.

You begin this level in a small room near the northwest
corner of the map.  You'll be facing south, towards a
doorway.
Step through this doorway into the wide rectangular room
beyond.  This room streches away east and west, and there
are 4 windows in the south wall.
Turn left (east) and walk a few paces, until you come to
the last window on your right.
Turn right to face south, and look out that window... You
should see the curving wall of a balcony just outside the
window and a bit to the left.  This balcony sticks out into
the east side of a courtyard, which is just outside and
beneath the window.
Step back a pace or two from the window to get a running
start, then Run south, out through the window, to land upon
the balcony.  (Remember that "Run" in PC Doom/Doom II games
is typically enabled by holding down the Shift key.)
If you don't make the jump, or fall off the balcony, go to
the middle of the courtyard and look north for an enclosure
where you'll find an elevator.  Use it to go back up to the
window, and try the jump again.
Once you're on the balcony, turn left (east) and step into
a small room.  (Beware of ambush here: As you leave the
balcony behind you, the wall ahead of you to the east will
open, releasing one or more monsters.)
Turn left to face the north wall of this room.  This wall
can be Activated like a switch.  (When activated, it opens
up momentarily; and, simultaneously, a floor panel will
drop down to the north...)
Activate that wall to open it, and Run north, to the next
wall straight ahead of you.
(While running, stay away from the wall on your right, or
you may step into a floor trap.)
Look for an opening in the floor, right along the bottom
edge of that wall ahead of you to the north.
Step down into that opening, and continue moving north
before the floor panel rises back into position.
If you didn't make it in time, turn around and look for the
window from where you can jump back to the balcony.
Once you've made it down through the floor opening, you'll
be in the secret room where the plasma rifle is.
There's a switch on the south wall of this room.  Activate
it to drop the floor panel/elevator again, and ride it up
to exit this area.

Done!

Note that once you have the red key, you can gain access to
the rectangular room that's just southeast of the balcony.
On the north wall of this room you'll find a narrow window,
and a torch just to the right of it.  This torch can be
Activated like a switch.  When Activated, it opens that
same wall to the east of the balcony, and opens the same
floor panel that allows access to the secret room... If you
move fast enough, you can Activate the torch, and Run north
through the narrow window, past the open wall, and get down
into the secret room before the floor panel rises again.

To read about or download a demo of this level, showing the
two ways to access the plasma rifle, click here.