To access all "official" Secrets and Items in PSX Final
Doom level 28, Baron's Lair:

From the level's starting point in the central courtyard,
enter the small brown door to the west (Automap west).
Go down into the mud pit ahead of you, and Activate the
switch you'll find there.
The walls to your left and right will drop down, uncovering
another switch to the northwest, just around a corner.
Activate that switch and go to the southwest corner of the
room.  You'll see a door there.
Open the door, go up the elevator and grab the blue Soul
Sphere.
Go back down and go back around to the northwest corner of
the room.  Go through the open door there, and follow the
stairs up and around to the red key (out and to your
right).
Grab the key and step on one of the nearby teleport pads to
get back to the door that leads to the central courtyard.
From the courtyard, go through the red-keyed door.  Then,
go down into the wide pit ahead of you to find the blue
key on the ground.  Grab it and go back to the top edge of
the pit.
In the southeast and southwest corners of the room you'll
find control panels recessed into the walls.  Shoot these
panels with your pistol, shotgun or chaingun and they'll
open, revealing secret areas containing some ammo.
Grab the ammo and go back to the courtyard.
Go through the blue-keyed door and you'll find a switch on
the ground ahead.  Activate this switch to lower the wall
in front of you, revealing a long north-south chamber.
Go south until you reach the far wall, then turn around to
face north.
You'll find switches to your left and right, recessed into
the backs of two supporting structures.
Activate both switches, then go to the small pool in the
center of the room where you'll see a Berserk Pack on a
large pedestal.
Step up to the core of the tallest pedestal and Activate it
like a switch.  4 smaller pedestals surrounding it will
drop down into the pool.  Step on each of these to get
credit for discovering a Secret.
Pick up the Berserk Pack and head back out to the central
courtyard.
Go up onto the platform in the center of the courtyard, and
Activate a switch you'll find on the west side; this opens
a secret area in the northwest corner of the courtyard.
Another switch on the south side of the platform can be
activated to open another secret area in the southeast
corner of the courtyard.
You'll find that the wide door on the east side of the
courtyard is now open.  Go through that door and take note
of the two pedestals that are up ahead and to your left and
right.  (Revenants may be standing on these pedestals.)
Continue east until you get to the stairs ahead.  Up the
stairs and to your left is a switch; Activate it.
Go back down the stairs and go right, into a newly-opened
section, and walk west.  You'll see a switch in the corner.
Activate it to open up a new southern section of this room.
Go south into the new section, and look for a switch in the
middle of the southern-most wall.
Activate that switch and QUICKLY run out to those pedestals
where the Revenants were standing.
The switch you just activated opens up the two pedestals,
and inside each pedestal is another switch.
The pedestals remain open for only a couple of seconds, and
then close up again.  You'll have to activate each of these
pedestal switches in order to open up a path to the exit.
Once you've activated both switches, go back up the stairs
to the east, and you'll see that a door has opened to your
left... Go through and follow the path around to the exit.