Medieval Information

Digging was very common when storming a castle. It was so common, that newer castles had to incorporate subterranean defenses as an attempt to stop such attacks.

Unfortunately for older castles, diggers could very frequently destroy walls and towers in a couple of weeks of digging. Diggers were employed when storming a castle by a different approach was impossible - this happened very frequently because of a hurry as isolating a castle would take much more time.

Since digging could potentially destroy a huge castle wall, this method was avoided by the castle attackers because what they normally sought was to take the castle for themselves.

When digging did take place, the defenders had very little choices. The most common one was to make a counter-tunnel - this consisted in making a tunnel which would intercept the opponent. When such an interception did occur, a fearsome underground struggle would take place until the tunnel was itself destroyed and another one had to be built.

Ways:

After the tunnel was built, the most common practice was to simply let it collapse along with the castle's wall.

Another way was to simply make a small tunnel and let the invaders enter from it (to avoid castle damage).

Either way was extremely feared by the defenders as they could only watch.

Detection:

The defenders would do whatever they could in order to detect an enemy's tunnel. What was normally done was pretty simple - water was poured into the ground to see if it drained hastily. If it did, there very likely was a tunnel.

Unfortunately for the defenders, water was very often scarce and thus; another defending weakness.

Rain:

Rain was the defender's best friend. It supplied them with water whilst potentially destroying any ongoing tunneler progress.

Of course, everybody loved rain - except the attackers that is.