To get the Soul Sphere in Doom II level The Citadel:

From the central courtyard in the middle of the map, walk
south through the door and turn left.
Go through door ahead in the east wall and turn right.
Activate the switch in the corner, and a small set of steps
will emerge from the floor.
Go up the steps.  At the top you'll see a wide door in the
east wall, and to the left of this door you'll see two
face-like panels on the north wall.  Step up to the blank
wall panel in between those faces, and Activate it like a
switch.  It opens to reveal a stairway.

Go to the top of the stairs, and look down into the north
end of the room.

In PlayStation/Saturn Doom, there will be a wide pedestal
visible in the middle of the floor below.  In PC/Mac/GBA
Doom II, that pedestal will be recessed into the floor, and
must be raised by activating a nearby switch 2 or 3 times
(the switch on the wall that faces towards the stairway).

Jump down to the pedestal, then Run to get on the triagular
platform in the northeast corner of the room.  (If you miss
the jump, look for a secret panel in the southwest corner
of the room.  Activate it to access a passage leading back
to the central courtyard.)

Once you're on the triangular platform in the corner, step
into the nearby teleporter booth, and you'll materialize at
Soul Sphere's coordinates.

PC Doom II players can click here to access a demo showing
a way to access the Soul Sphere.