I'm finally actually participating in the weekly topic challenges, instead of just helping run them.
Let's say that there's a siege around a small series of fortifications that comprise the only defense of a small band of rebel soldiers (the technology level is about that of Europe circa 1100). The surrounding army can't seem to get in because the rebels have chosen to hide amidst old fortifications, like the ones at some places along Hadrian's Wall. They're old even at this point in time, but instead of some of the fortifications at present-day Hadrian's Wall, the fortifications are ten feet tall and about two feet deep.
The soldiers have no weapons that they can use to get in. They can't go over the wall, because there's a two-foot deep ditch surrounding the wall that increases the effective height to about twelve feet. They have no rope, and only bows and arrows, swords and shields. There are no trees of other vegetation which could help them - in fact, no natural materials they could use at all. Yet they need to quickly squelch out the rebels (i.e. within a couple days, at which point they have to leave and fight elsewhere), who have enough provisions to survive for two weeks.
Fortunately, the soldiers know that the surrounding countryside is filled with a certain kind of insect that can eat through stone. They spend all but one of their remaining days catching enough of these insects. They gather them in bags and then let them loose at the base of one of the sections, where the insects manage to carve a hole big enough for an armored man to get through. The rest is history.1
Could these insects exist? If so, would it be reasonable for a large swarm (e.g. 5,000 - the soldiers were busy) to eat a hole of this size in a period of 24 hours?
The surrounding region looks a fair bit like Northeast England, as shown here:
1 Not yet, actually. I don't know what happens next.