To access the Mega-Armor on Doom 64 map 30, The Lair:

When you first go underground, and get to the bottom of the
curving staircase, follow the left-hand wall until you get
to the blue Mega-Armor in a raised, square opening in the
wall.
Continue following this wall a few paces (passing the first
opening on your right), until you get to an intersection or
a fork in the path.

You'll be standing on a narrow stone walkway, at the center
of a 4-way junction: One path straight ahead which leads to
the yellow key, one path branching off ahead and to your
left, another branching off ahead and to your right, and of
course the path behind you that you just came from.

Take a step off this walkway and into the right-hand
passage, and look along the right-hand wall for a cubbyhole
where a stone gargoyle sits (this is the first of several
gargoyles lining the walls of these passages).
Activate that particular gargoyle like a switch.  Then turn
and step back onto, and over, the narrow walkway in that
4-way junction, to head into the opposite passage that
curved off to the left.

Follow the left-hand wall here, as it curves around to a
new opening.  Step in and follow the tunnel to where the
armor sits at the far end.