Sapper

A sapper is an individual in military service, often called a "military engineer" nowadays, that 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 in weakening 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.