Get chaingun/backpack in Doom 64 map 14, EYE OF THE STORM:

From your starting position at the beginning of the level,
look for the 2 stairways that lead up out of the water.
The backpack sits on a battlement to the right of the
right-hand stairway, the chaingun is on a battlement off to
the left of the left-hand stairway.

To get to either battlement, go up the nearby elevator so
that you're on the highest level above it, then step up
onto the wall, and prepare to Run (remember your Run
button) along the wall to jump to it.
A useful trick here is to Strafe-Run, using a simultaneous
combination of Run-forward plus Sidestep.  This will move
you at an odd angle, but provides a little more speed than
straight running does.

Hope this helps. :)

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