Also powder magazine . Magistral line . The base line from which the various parts of a fortification were traced. A movable shield or screen made of heavy planks to protect besieging or besieged soldiers from musket fire. The solid feature between embrasures in a parapet, or in older fortifications, the solid part of a wall between crenels from which archers defended a castle.
Mortars are cannon suitable only for the high-angle fire of shells.
Their primary function was to drop a shell or bomb behind earthworks or other defenses which could not be penetrated with gun fire. Mortars fired hollow iron spheres or shells filled with small shot, balls or similar anti-personnel devices combined with a small charge of powder which was ignited by a time fuse designed to explode just as it reached the target. The usual length of a mortar was 3 or 4 calibers and the trunnions were located at the rear of the breech. An outer defense, inside the glacis but outside of the body of the place. A ravelin is an outwork.