Dark tower 'invisible gap' glitch

When you enter the Dark Tower, look at the entrance towers. The one on the right (as seen from the inside), has another tower attached diagonally to it - and THAT tower (the one that has a container on the top floor) has a gate on its ground floor that cannot be unlocked and a space behind that. If you climb down from the upper level of that tower to the mid level you can fall through a gap that cannot be seen (at least I can't see it) to the ground floor, where you are stuck on the wrong side of the gate you cannot open, and you cannot jump or climb back up again.

Fortunately for me, I had just had a conversation with Jonathan (which creates an auto-save), so I didn't lose much time.