...because it's just a casemate. Sure, it wouldn't be a particularly pleasant place to pass the time, but it's not a dungeon by any stretch of the imagination. There's a big ole hole in the wall to the right, which opens right into the main gate. One imagines that was unfortunate for those passing through the gate, as a prisoner could notionally say rude things or throw feces at them through this hole. During a bombardment, anyone imprisoned in this deadly hole would be the safest dude in the fort.