Glacis

A glacis ( /ˈɡleɪ.sɪs/; ) in military engineering is an artificial slope of earth used in late European fortresses so constructed as to keep any potential assailant under the fire of the defenders until the last possible moment. On natural, level ground, troops attacking any high work have a degree of shelter from its fire when close up to it; the glacis consists of a slope with a low grade inclined towards the top of the wall. This gave defenders a direct line of sight into the assaulting force, allowing them to efficiently sweep the field with fire from the parapet. Additionally, but secondarily, the bank of earth would shield the walls from being hit directly by cannon fire.