Aunis is a historical province of France, situated in the north-west of the department of Charente-Maritime. Its historic capital is La Rochelle, which took over from Castrum Allionis (Châtelaillon) the historic capital which gives its name to the province.It was a fief of the Duchy of Aquitaine. It extended to Marais Poitevin in the north, Basse Saintonge (and Niortais) in the east, and Rochefortais in the south.