An engineering design incorporated in castle design, these were artificial slopes built into defensive walls or structures. They extended outwards away from it's foundations. Made of earth or sand - some could even be constructed of rock. Often smoothed, they began to resemble hill or glacier (hence the French, Glacis). They also functioned as a form of buttress support. Their design made it difficult for troops to climb as well as getting siege device close enough to the walls of the structure. Another innovative design of the Glacis, their slopping construction allowed defenders to drop rocks, debris etc from the top wall. Incorporating a change in the momentum of the falling object - bounce was created. Thus the "projectile" angle was changed. With the advent of artillery, Glacis were effective at absorbing shell impact. Through structural evolution, defense towers (built above them), would slowing be made flush with the Glacis. Thus, the creation of the reinforced gun platform. Crak de Chevaliers has classic examples of Glacis incorporated into it's design.