Cheval de frise
The cheval de frise (, plural chevaux de frise ; French pronunciation: [ʃə.val də fʁiz], plural [ʃə.vo də fʁiz], "Frisian horses", not to be confused with the animal Friesian horse) is a defensive obstacle, existing in a number of forms, principally as a static anti-cavalry obstacle but also quickly movable to close breaches. The term was also applied to underwater constructions used to prevent the passage of ships or other vessels on rivers.