Access the secret exit in GBA Doom II level 16, Industrial
Zone: B, which leads to level 33, Wolfenstein:

When you first appear on the Industrial Zone: B map, you'll
be facing west, looking down a hallway towards a square
demon-faced panel that's high up in the far wall.
Just ahead of you near your starting point is an opening to
a stairway on your left.  Go up the steps, and as you get
to the top, turn left to face a brown metallic wall panel.
This is an elevator; Activate it, get on and ride up.
At the top, turn left (east), and you'll be facing a torch.
On your left is a stone wall.  Follow this wall on your
left past the torch and around the corner (south a few
steps, then east, then north) until you get to a teleporter
pad between two more torches.
Step on the teleporter and you'll materialize on another
teleporter at one end of a short hallway.  At the other end
is a switch on the wall; Activate it and step back on the
teleporter.
You'll materialize back on the first teleporter.  On your
right is the stone wall.  Follow that wall back around the
corner to the elevator you rode on a minute ago.
Go back down the elevator.  At the bottom, turn east and go
back down the stairs.
At the bottom of the steps, walk around the corner on your
left, and walk west a few paces until you get to a narrow
bridge on your right.
Go across the bridge (north) and go left.  At the corner go
left again, then go up the steps (south) to a wooden door.
Open the door and grab the yellow key you'll find there.
Go back down the steps, and at the bottom look ahead and to
the left (north/northwest) for a teleporter in the corner.
Walk up and step on.
You'll materialize on another teleporter pad outdoors on
the grass.  You'll be facing northeast, and your back will
be towards a corner formed by tall stone walls.
Walk a few paces east, to where the grass ends at the edge
of a low river of crusted lava.
Face east towards the widest part of the lava river, and
you should see a small rectangular pedestal jutting up from
the lava below.  On top of the pedestal will be a radiation
suit, and just behind the suit is a switch mounted on a
small vertical slab.
Getting onto the pedestal and Activating that switch is the
next objective...
Turn left a little to face northeast, and you'll see tall,
greenish stone walls rising up from the lava below.  There
is another teleporter pad located down in the lava where
these stone walls come together at a corner.
Jump down to the crusted lava, and run onto the teleporter.
You'll materialize back near the top of the elevator you
rode a few minutes ago.  You'll be facing south, looking
towards where the checkered floor ends at a high dropoff.
From where you stand now you can run south off the dropoff
and land on the pedestal where the switch is.
This can be tricky... It's easy to run too far, or not far
enough, and land down in the crusted-lava river.
If you miss the pedestal or fall off, turn right/northwest
and follow the greenish stone walls, keeping them on your
right, until you get to that teleporter in the lava where
the walls form a corner; Then try again for the pedestal.
Once you land on the pedestal, Activate the switch.  This
opens a passagway at the far east end of the crusted-lava
river.  Jump down and run northeast and east to that end
of the river.
Enter the wide opening and follow the passage around to a
chamber where you'll see an Invulnerability Artifact (a
green sphere) on a wide, low platform in the lava.
The Artifact itself rests upon a flashing stone rectangle.
Stepping onto or across this flashing rectangle will open
a secret-exit passage at a distant location on the map.
(That passage will open whether or not you pick up the
object, just so long as you step onto the flashing stone
rectangle where it rests.)
Step onto the flashing rectangle and pick up the object if
you wish; then run back out across the river, and run back
to the teleporter in the lava, and step on.
Again you'll materialize back by the top of the elevator,
and you'll be facing south, looking towards where the floor
ends at a high dropoff.
Walk a few paces south, to the end of the dropoff, and turn
right to face west.  In the distance you'll see where stone
battlements rise up from the grass below.
Run west off the dropoff, through the air above the lava to
land on the grass.  (If you fall down to the lava, run back
to the teleporter and try again.)
Continue northwest on the grass (keeping the river to your
right, battlements on your left), until you get to a
yellow-keyed door between two torches on your left.  Go
south through that door.
Around the corner on your right, just beyond a flickering
torch in the corner, you can find a radiation suit in a low
opening in the wall, where one end of a narrow crusted-lava
stream begins.  Pick up the suit now if you wish.
Follow the lava stream to the far end of the room, and then
through the low, dark opening in the south wall.  You'll be
entering into a dark curving passage.
Follow the left-hand wall of this passage until you come to
a new, glowing tunnel on your left.  Step into that tunnel
and follow it around to a teleporter in a small chamber.
Step on that teleporter to exit to the Wolfenstein level.