Tunneling was an effective way to breach a castle's defenses.
Men called sappers sometimes dug tunnels to gain entrance to a castle and thereby launch a sneak attack, but more often, these miners dug tunnels beneath a castle wall to destabilize and topple it. They supported their tunnels with timbers, which they then burned to collapse the tunnel—and, hopefully, the wall above.
To defend themselves, castle dwellers put out a bowl of water and watched for ripples that might indicate digging. Sometimes the castle's garrison built counter-tunnels; if the two tunnels met, fierce battles ensued underground.