A sapper, in the sense first used by the French military, was one who sapped another's fortifications. A sapper is an individual in military service, often called a military engineer nowadays, who may perform any of a variety of tasks, under combat conditions. Such tasks typically include bridge and road construction; and mine laying, or detection and clearing. In other words, the sapper's tasks now involve facillitating movement of allied forces, and impeding movement of the enemy's.
When castles served a military purpose, one of the tasks of sappers was to weaken the bases of castle walls to enable them to be breached--before means of thwarting these activities were invented. Broadly speaking, sappers were originally experts at demolishing or otherwise overcoming or bypassing fortification systems.