glacis

Glacis comes from the Old French glacier (to slip, to slide), from the Latin glaciēs (ice). The original sense was a slippery slope. In military engineering, a glacis is the long, gentle slope extending outward from the outermost ditch of a fortification. It looks like nothing — just a grassy incline. That is the point. The slope is carefully graded to ensure that anyone crossing it is fully visible to defenders on the walls, with no cover and no dead ground.

The glacis was perfected by Vauban in the late seventeenth century. His fortress designs at Neuf-Brisach, Besançon, and Lille included mathematically precise glacis slopes calculated so that the line of fire from the ramparts grazed the surface. A defender standing on the wall could shoot at anyone on the glacis from a considerable distance. The attacker had to cross an open, exposed killing ground under sustained fire before reaching the ditch.

The concept extended beyond individual fortresses. The Glacis of Vienna — the open ground surrounding the old city walls — was maintained as a clear field of fire until the walls were demolished in 1857. The Ring Road (Ringstraße) was built on the former glacis. Some of the most expensive real estate in Vienna sits on ground that was kept empty for three centuries because it was a killing field.

In modern military terminology, 'glacis plate' refers to the sloped front armor of a tank. The principle is the same: an angled surface deflects incoming fire. The gentle slope that exposed medieval attackers became the angled plate that protects modern tank crews. The word kept the physics and changed the direction of protection.