A sapper, in the sense first used by the French military, was one who dug trenches to allow besieging forces to advance towards the enemy defensive works and forts, over ground that is under the defenders' musket or artillery fire. This digging was referred to as sapping the enemy fortifications. Saps were excavated by brigades of trained sappers or instructed troops. When an army was defending a fortress with cannons, they had an obvious height and therefore range advantage over the attacker's guns. The attacking army's artillery had to be brought forward, under fire, so as to facilitate effective counter-battery fire.
This was achieved by digging what the French termed a sappe[1] (derived from the archaic French word for spade or entrenching tool).[2][3] Using techniques developed and perfected by Vauban, the sappers began the trench at such an angle so as to avoid enemy fire enfilading the sappe by firing down its length. As they pressed forward, a position was prepared from which a cannon could suppress the defenders on the fort's bastions. The sappers would then change the course of their trench, zig-zagging toward the fortress wall. Each leg brought the attacker's artillery closer until the besieged cannon would be sufficiently suppressed for the attackers to breach the walls. Broadly speaking, sappers were originally experts at demolishing or otherwise overcoming or bypassing fortification systems.
A sapper, in the sense first used by the French military, was one who dug trenches to allow besieging forces to advance towards the enemy defensive works and forts, over ground that is under the defenders' musket or artillery fire. This digging was referred to as sapping the enemy fortifications. Saps were excavated by brigades of trained sappers or instructed troops. When an army was defending a fortress with cannons, they had an obvious height and therefore range advantage over the attacker's guns. The attacking army's artillery had to be brought forward, under fire, so as to facilitate effective counter-battery fire.
This was achieved by digging what the French termed a sappe[1] (derived from the archaic French word for spade or entrenching tool).[2][3] Using techniques developed and perfected by Vauban, the sappers began the trench at such an angle so as to avoid enemy fire enfilading the sappe by firing down its length. As they pressed forward, a position was prepared from which a cannon could suppress the defenders on the fort's bastions. The sappers would then change the course of their trench, zig-zagging toward the fortress wall. Each leg brought the attacker's artillery closer until the besieged cannon would be sufficiently suppressed for the attackers to breach the walls. Broadly speaking, sappers were originally experts at demolishing or otherwise overcoming or bypassing fortification systems.
https://en.m.wikipedia.org/wiki/Sapper