35. Tunneling Beneath Castle Walls
Besiegers often dug tunnels to defeat castles and other fortifications. Tunneling was particularly effective against walls that were not built atop solid rock. Besiegers would dig beneath the walls either to bypass them and allow attackers to emerge on the other side, or to undermine and collapse the walls.
When undermining the walls, besiegers would tunnel until they got beneath the foundations of a wall section. As they excavated a space beneath the foundation, they would use temporary wooden props to keep the walls up. Once a sufficiently large space was dug beneath the walls, the besiegers would burn the props, causing them to collapse, along with the section of the wall above them. The defenders, always on the lookout for such attempts, often dug counter tunnels in an attempt to intercept and destroy the underground attackers.