Li Shenron
Legend
To clarify - the physicality of a locale is less important than the psychological sense of safety/security. It's one thing to border up the doors and windows and another to actually relax. On the flip-side, they can believe a place is safe, begin to rest and then find out they were mistaken.
I was about to post the same...
I have easily allowed short rests inside a dungeon, but I wouldn't do so with goblins banging on their door, even if it's barricaded.
My "trick" when I am not sure between allow and disallow, is to suggest that a couple of the PCs should be on high-alert guarding duties (and they do not get the benefits of a short rest) in order to guarantee enough sense of safety for the other PCs (which get the benefits). Unless they are all low on resources, this is usually generous enough to be almost as good as a short rest for all, even without allowing two short rests in a row to switch guards.