Mines

One tactic commonly used in fortress sieges at this time was that of undermining. Specially trained groups of besiegers, known as sappers, would dig deep underground tunnels supported by wooden props, leading right under the fortress walls. When the order was given, combustible materials or explosives would be ignited, leading the underground passageways to collapse and the ground above to subside, dragging down sections of the wall above. The same tactic was often used by the defenders, either so as to neutralize the enemy sappers or to destroy fortifications built by the besiegers.