How to open the four central chambers in Unholy Cathedral:

In the large courtyard that dominates the center of the map
are four large, sealed chambers.  These can be opened by
crossing various invisible trigger lines, or "tripwires".

Around the perimeter of that wide courtyard are four large
teleporter pads.  Behind each of these pads you can see a
pair of sealed doors.

Three of those four chambers can be opened by tripwires in
the doorways behind those teleporters; The other chamber is
opened by crossing a tripwire around the teleporter in the
corrosive-floored room in the northwest corner of the map.

Specifically, clockwise from the northern-most chamber:
The north chamber is opened by stepping through either door
next to the courtyard's northeast teleporter.
The east chamber is opened by stepping through either door
next to the courtyard's southeast teleporter.
The south chamber is opened by stepping onto the raised
teleporter pad located at the far end of the corrosive room
in the northwest corner of the map.  (That room is opened
by stepping across a narrow trapezoid-shaped hole in the
floor, located just outside the south end of the room.)
The west chamber is opened by stepping through either door
next to the courtyard's southwest teleporter.

PC Doom players can click here to access a demo that shows
how to open all four boxes on this level.