Inaccessible Items/Secrets in level 20, UNHOLY CATHEDRAL:

In the main, central courtyard, there's a secret panel in
the east wall.  (It's to the lower right of the red, square
indentation high up in the wall.)  Behind this panel is a
small chamber which contains a Soul Sphere ("Supercharge")
and a chain saw.
Unfortunately, in PSX Doom this chamber is inaccessible due
to a flaw in how the map was translated from the original
PC Doom game.
In computer-based versions of the game, this secret panel
can be opened with the yellow key.
In the PlayStation version, this panel can't be opened with
the yellow key, or by any other means.  Because of this,
the maximum Secret score for this level will be under 100%.
For better or worse, this is normal for the PSX version of
this map.

--Led

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