Cathar Wars - Medieval Warfare - Mining, undermining defensive walls

A"mine" was a tunnel dug to destabilise and bring down castles and other fortifications. The technique could be used only when the fortification was not built on solid rock. It was developed as a response to stone built castles that could not be burned like earlier-style wooden forts.

A tunnel would be excavated under the outer defences either to provide access into the fortification or more often to collapse the walls. These tunnels were supported by temporary wooden props as the digging progressed, just as in any mine. Once the excavation was complete, the mine was filled with combustible material. When lit it would burn away the props leaving the structure above unsupported and liable to collapse.

To save effort attackers would start the digging as near as possible to the wall or tower to be undermined. This exposed the sappers to enemy fire so it was necessary to provide some sort of defence. Pierre des Vaux de Cernay recounts that at the siege of Carcassonne in 1209, during the Cathar wars (Albigensian Crusade),

Successful sapping usually ended the battle since either the defenders would no longer be able to defend and surrender, or the attackers would simply charge in and engage the defenders in close combat.

There were several methods to resist under mining. Often the siting of a castle could be such as to make mining difficult. The walls of a castle could be constructed either on solid rock or on sandy or water logged land making it difficult to dig mines. A very deep ditch or moat could be constructed in front of the walls, or even artificial lakes. This makes it more difficult to dig a mine and even if a breach is made the ditch or moat makes exploiting the breach difficult.

The defenders could also dig counter mines. From these they could then either dig into the attackers tunnels and sortie into them to either kill the sappers or to set fire to the pit-props to collapse the attackers' tunnel. Alternatively they could undermine the attackers' tunnel to collapse it.

If the walls were breached they could either place obstacles in the breach for example a chevaux de frise to hinder an attack, or construct a coupure.

The practice has left us reminders in English. "undermining" has acquired figurative as well as literal meanings. And military engineers are still known as Sappers.