Glacis

A glacis (/ˈɡleɪ.sɪs/; French: [ɡlasi]) in military engineering is an artificial slope as part of a medieval castle or in early modern fortresses. They may be constructed of earth as a temporary structure or of stone in more permanent structure.

A glacis plate is the sloped front-most section of the hull of a tank or other armored fighting vehicle.

More generally, the term glacis can denote any slope, natural or artificial, which fulfils the above requirements. The etymology of this French word suggests a slope made dangerous with ice, hence the relationship with glacier.

A glacis could also appear in ancient fortresses, such as the one built at Semna, by the ancient Egyptians. Here it was used by them to prevent enemy siege engines from weakening defensive walls.

Ancient British hill forts started to incorporate glacis around 350 BC. Those at Maiden Castle in Dorset were 25 metres (82 ft) high.

Glacis, also called talus, were incorporated into medieval fortifications to strengthen the walls against undermining, to hamper escalades and so that missiles dropped from the battlements would ricochet off the glacis into attacking forces.

Towards the end of the medieval period some castles were modified to make them defensible against cannons. Glacis consisting of earthen slopes faced with stones were placed in front of the curtain walls and bastions (towers) to absorb the impact of cannon shots or to deflect them. Towers were lowered to the same height as the curtain walls and converted into gun platforms.

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