To find the secret exit on Doom 64 level 1, Staging Area:

(Note: For a video demonstration of the walkthrough below,
click here.)

When you first appear on this level, take note of three
things in the immediate starting area: A barrel a little
ahead of you and to the right, another barrel further ahead
and in the left corner against the far wall, and a closed
door directly behind you.

The door behind you leads to a small room.  As you enter
that room, you'll see a Berserk Pack ahead of you, and on
the floor to your left is a small stain or puddle against
the wall.  The secret exit (a teleporter booth) is hidden
in the wall directly behind that stain.

This teleporter can only be accessed once per game, and it
only stays open for a few moments, then permanently closes.
The teleporter opens when all of the barrels on the level
are destroyed.  There are 10 of them, and a couple of them
may be hard to find.  (Read BARRELS, below, for their
locations, or click to view a map of barrel locations.)

As the last barrel explodes, the exit booth opens up, stays
open for about 5 seconds, and then closes.  Once closed,
it's closed permanently; Therefore, you want to be able to
get to the secret exit quickly after destroying the last
barrel.  You can choose among a few different barrels to be
the last one; but the easiest thing to do is to save the
barrel that's closest to the secret exit for last.

When the other nine barrels have been destroyed and you're
ready to leave, open the door to the small room where the
secret exit is, then stand in the doorway so the door can't
close.  Use your pistol or shotgun to fire at the nearby
barrel ("barrel 10") that you've saved for last.  You can
shoot through the wire-mesh cage blocking that barrel.

As the last barrel explodes, turn and Run into the secret
exit teleporter, and you're off to level 32...

BARRELS
-------

Barrel 10 (save for last):
There are 10 barrels in this level.  Two of the barrels are
in the starting area, as mentioned above.  For best results
save the barrel closest to your starting position (nearest
to the secret exit) as the last/10th barrel.

Barrel 1:
When you first appear on the level, barrel 1 is in the
starting area (along with barrel 10), and is in the far
left corner of the large square space just ahead of you.

Barrel 2:
Walk out of the starting area, past barrel 1's location,
turn left, and walk until you see a door on your left.
Enter that room and barrel 2 is in there.

Barrel 3:
Step out of that room and turn left and walk to the end of
the hallway and turn right.  Walk all the way to the end
until you're at a triangular switch.  This switch lowers a
lift; Get on and ride up.
When you get to the top you'll be looking down a long
hallway, with short hallways branching off on either side.
From this position, barrel 3 is behind you on your left.

Barrel 4:
From the top of the lift that you just rode, walk forward
all the way to the end of the long hallway, and take the
last left, then left again.  On your left will be a secret
panel.  (Almost all of the walls in the area have twin rows
of circles running horizontally across them; the secret
panel is missing these circles.)
Open that panel and barrel 4 is in there.

Blue key:
In this general area of the level (where barrels 3 and 4
are found), there is a small niche with a switch on its
back wall.  At lower Skill levels the blue key is on the
floor near this switch; At higher Skill levels you'll need
to activate the switch and quickly Run to a small closet
that temporarily opens nearby.  (As you face the switch,
the closet is around the corner behind you on your left.)

Barrel 5:
Once you have the blue key, go back to the lift, then drop
down off the lift to fall back into the hallway you came
from a while ago.  Walk forward until you come to the
blue-keyed door on the left.  Walk in through the door and
you'll see barrel 5 ahead of you across the room.

Barrel 6:
In the middle of the room (where barrel 5 was) is a wide
pillar and a small stair-step in the corner, leading up.
Go up and head left to the end of the long passage, where
you'll find a switch on the wall to your left.  Activate it
to open a large room behind you.  In this room is another
switch on the far wall (activate it) and barrel 6.

Barrel 7:
After activating the switch where barrel 6 was found, walk
back out of that room and turn left.  Go back down the
corridor until you come to a new opening on your left, and
step through.
Follow the left path around to a small alcove on your left.
Walk in there and the floor drops down.  Just ahead is a
tiny room below floor level, where you'll find several
health potions, a special monitor screen on the wall, and
barrel 7.

Barrel 8:
Come back up out of the tiny room, turn left and proceed
down the path to the door ahead of you.  Open it and go
through into a large room.  At the far end of this room is
another door leading into a small chamber where the regular
exit to level 2 is.  In here is barrel 8.

Barrel 9:
From barrel 8's location inside the regular-exit chamber,
step out of that chamber into the large adjacent room, then
leave through the door at the far side.  Follow the curving
path past the tiny drop-down room where barrel 7 was found,
and continue to the next opening on your right with a wide
step leading up.  Go up and you'll see the room ahead where
barrel 5 was found, and you should be able to see the open
end of cubbyhole that's inside of a wide, floor-to-ceiling
pillar.  On the back wall of this cubbyhole is a switch.
Back up to get a running start and Run into the cubbyhole.
When you activate the switch, it temporarily lowers a lift
in the corner of the room (to the right of the blue-keyed
door).  The lift drops down beneath floor level, opening a
path below.
Activate the switch and quickly back out of the cubbyhole
and look slightly to the right.  If you can make it to the
opening in time, jump down in, and in the lower passage
you'll see barrel 9.

Barrel 10 and the secret exit:
When barrel 9 is destroyed you can head back to the upper
section of the level's starting area.  To get there you'll
need to use one of two teleporters:

There's a teleporter that's just around the corner in the
tiny passage where barrel 9 was.  Once that barrel is gone,
this teleporter can be used at any time (and repeatedly) to
reach the upper section of the starting room.
Alternatively there is a hidden (but not Secret) teleporter
that's located just outside and directly across from the
blue-keyed door.  Note that this teleporter can only be
used once, and using it can cause Shotgun enemies to spawn
into the starting area.

When the other nine barrels have been destroyed and you're
ready to leave, open the door to the small room where the
secret exit is, then stand in the doorway so the door can't
close.  Use your pistol or shotgun to fire at the nearby
barrel ("barrel 10") that you've saved for last.  You can
shoot through the wire-mesh cage blocking that barrel.

As the last barrel explodes, turn and Run into the secret
exit teleporter, and you're off to level 32...

(Note: For a video demonstration of the walkthrough above,
click here.)