ELI5: Explain Like I'm 5

Glacis

Glacis is a special type of landform. It is a gently sloping area of land where the ground gets gradually higher. Glacis is usually formed when a river carves into the landscape. This creates a kind of natural ramp that connects two areas. Glacis is most often found around old fortifications (like castles) where it helped to protect the castle from attack.
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