Glacis

A glacis is a broad, gently sloped earthen bank placed in front of a fortress or other defensive work. Its primary purpose is to hinder and expose attackers by creating an open approach that defenders can observe and fire across while reducing the impact of direct fire on the wall behind it. The slope also helps to deflect or absorb artillery energy and to impede mining or sapping of the ramparts. In traditional European fortifications, the glacis extends outward from the parapet or ditch and blends into the surrounding terrain, often incorporating drainage to prevent water accumulation.

Construction and form: Glacis are typically built from compacted earth, sometimes with a facing of rubble or masonry, and are graded to maintain a uniform slope over a considerable distance from the fortification. The geometry varies with style, but the common aim is to keep attackers exposed as they advance and to provide defenders with a clear line of fire across the approach. The glacis is usually integrated with other outer works such as ravelins, tenailles, and counterguards as part of an overall field of fire and protection system.

Historical context and usage: The glacis is a central element of early modern fortification, associated with the designs of Vauban and other engineers in the 17th and 18th centuries. Its effectiveness depended on disciplined maintenance and precise leveling, and it contributed to the strategic logic of concentric defense by increasing the time and danger faced by would‑be attackers.

Modern relevance: While the formal glacis is less common in contemporary fortifications, the concept—using terrain modification to favor defenders—remains a foundational principle in military design and terrain shaping.