Missing secrets on level 29, The Death Domain:

In PSX Final Doom, two maps have "Secret" areas which are
inaccessible to the player.  The player character cannot
physically step into these areas, and therefore the game
will not count those secrets as being discovered.

On map level 29: The Death Domain, there is a short, small,
bracket-shaped passage on the west edge of the map.  This
passage is considered a "secret area", and counts as 50% of
the map's "official" Secrets.  The passage was intended to
allow access to an area on the southwest corner of the map,
but a door that blocks access to this passage cannot be
opened.  (The items at the southwest corner of the map
include 1 blue Mega-Armor, 3 boxes of rockets, and 4 boxes
of shotgun shells.  Also, the bracket-shaped passage
leading into this area contains 11 blue health potions.)

In the computer-based versions of Final Doom, the door to
this passage is opened by activating a switch that is built
into a demon-faced panel found in another bracket-shaped
passage at the northwest corner of the map.

In PSX Final Doom this demon-faced panel/switch is missing,
and therefore the door to access the passage which leads to
the southwest corner of the map cannot be opened.

Note that there are other hidden areas in THE DEATH DOMAIN,
but these are not counted as official Secrets.  (The other
"official" secret area of this level is near the northeast
corner of the map, where a blue Soul Sphere is found.)

100% Secrets can be attained on all other maps in PSX Final
Doom, with the exception of map 9: Nessus.