rideau
A fortification or barrier such as a small earthen mound or ridge, a file of troops, etc.
Rideau. A small rising ground or eminence, commanding a plain, which is sometimes near parallel to the works of a place. It is a great disadvantage to have Rideaus near a fortification, especially when they shoot from far, and terminate on counterscarp; for they not only command the place, but likewise facilitate the enemy's approaches.
Where it comes from
Borrowed from French rideau (“curtain”), from Middle French rideau, from Old French ridel (“plaited stuff; cloth curtain; blind”), from rider (“to plait; to wrinkle”), perhaps from Old High German ridil (“a headband; a ribbon”) or rīdan, wrīdan (“to wrinkle”) (related to English writhe), from Proto-Germanic *wriþilaz, from *wrīþaną (“to twist, wind, wrap”). See more at riddle (“curtain”), writhe.
Adapted from the Wiktionary entry rideau (CC BY-SA 4.0).