How to get to, and escape from, the "BFG room" in PC Final
Doom TNT level 20, Central Processing:

TO FIND THE BFG 9000
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You start the level near the middle of the western half of
the map, at the south end of a hallway.
Walk north to an intersection, then turn right and proceed
east to a metal door ahead.

Go through the door and turn left.  Head north into an area
dominated by metallic desks, chairs and partitions.
In the middle of this area, on the other side of the main
partition line, is a red teleport pad on the floor.  Go to
it and step on.

You'll materialize in a room to the south of the partition
area.  You'll be facing south towards a box.
Walk south a few paces, passing to the left of that box.
Up ahead and just to your left will be a line of boxes on
the floor.  The boxes run from a small box on the floor to
a series or larger boxes to the west.

Walk up these boxes like a stairway.  At the top of the
"stairway", turn left to face the corner of a large box
that's as high as the room itself.  Activate this upper
right panel of the box and it opens.

Step north into the box.  You'll drop down a bit to floor
level.
Step directly ahead and Activate the north wall of this box
from the inside, and it opens.
Step through the opening and continue ahead (north) to the
northeast corner of this room.

Here you'll see a squarish brick column in the corner of
the room, stretching from floor to ceiling.
Step up and Activate this column like a switch, and it
drops, allowing you to get on and ride up when it ascends
again a moment later.

At the top, look for an opening to the north.
This opening leads to an area that is laid out in a rough
"7"-like shape.  The perimeter of this area has a corrosive
floor; be careful not to touch it.
Running along the center of the floor is a low trough; you
can walk here without being burned.
Step into the trough and turn right.  Follow the trough a
few paces east, then south to the BFG 9000.

TO ESCAPE FROM THE BFG ROOM
---------------------------
The BFG is perched at the top of a small pyramid.  South of
the pyramid is a room with a teleporter and a switch.
The teleporter is in a box-like hole, recessed into the
south wall of that room; Above the teleporter, on the back
wall of the recessed box, is the switch.
You'll need to get into that room in order to leave this
area.

Between the BFG and that room is a window.  The lower edge
of this window is initially too far above the floor; you
won't be able to simply step through.
There are two main ways to get through the window into the
room beyond.

The recommended way is to stand where the BFG is perched,
and use your pistol to fire one bullet south, at the switch
above the teleporter.  The bullet impact will activate the
switch.  This lowers the window edge a bit, and also drops
the teleporter down near floor level.

It is also possible to get through the window before it is
fully opened, by using a simultaneous combination of Run
plus Strafe to run south, up and across the pyramid.
(This "strafe running" maneuver yields a speed slightly
above that of plain running, but moves the player at an
odd angle.)

If you get into this room before the switch is activated,
you can be trapped there.  In this case, you may still be
able to activate the switch by using the double-barreled
combat shotgun.  This weapon's pellet spread can sometimes
reach the switch if the player is standing far enough away,
at the north end of the room.

Once the teleporter is lowered you can simply step onto it.
You'll materialize at the south end of a room which is
located just south of the room where the boxes were stacked
in stair-step fashion.

To read about and/or download a demo that shows how to get
to the BFG, and how to escape from that area, click here.