Official Secrets in Doom 64 level 13, Dark Citadel:

There are only 2 Secrets on this map which will contribute
to the player's Secrets score.  Here's how to find them:

From your starting position at the beginning of the level,
take a sharp left and head down the hallway.  Follow the
hallway until it makes the first turn to the right.

Secret 1: At the corner, search the outer walls for a
secret panel, which opens to reveal a small chamber.  Step
inside and grab the items there.

Step back out and continue along the hallway, and step into
the large open room on your left.  This room is full of
narrow pillars, running floor-to-ceiling.  In the center of
the room is a pillar which is slightly wider than the
others.  On the back of it is a switch; Activate it (this
opens a door that you'll need to go through in a minute).
Go back to your starting position; and now follow the main
hallway in the other direction, until you come to a door on
your right.  Step through the door and take a sharp left.
Follow the left-hand wall around the corner until you get
to a short passage with some pulsing vicera on the floor
against the left-hand wall.

Secret 2: Activate that wall panel like a switch and it
opens, revealing a stairway leading down to some long,
narrow corridors located between the walls of some of the
larger rooms.  Down here you'll find a BFG 9000, and a
Megasphere.