Inaccessible Ledge in DOOM 2
Solution 1:
It turns out that you have to use the door to the tower—the wood-paneled wall to the left of the screenshot. (I didn’t even notice that it doesn’t close all the way because the top is not visible.)
You have to open the door, stand on it, then wait for it rise, taking you up along with it, and then run across the gap.