As shown in the movie Ironclad, they burnt a group of fat pigs together, under a tunnel, beneath the fort base and it resulted in a massive explosion.

From chemistry's perspective, and considering the tunnel environment — less oxygen, less space for energy to dissipate, would this really cause an explosion so massive that it will dismantle the structure above the tunnel? Or would there be no explosion, just burning?

Apologies for cruelty involved in the question.