The Viscountcy Castillon was situated in south-western France. At its centre was the town of Castillon-sur-Dordogne (now Castillon-la-Bataille). The purpose of the viscountcy, which has existed since the 10th century, was in the defence of the crossing over the River Dordogne. The family of viscounts lost their power because of a rebellion of Baron Guyenne against Simon de Montfort. At the end of the 13th century, Castillon passed to Jean I de Grailly, whose descendants ruled in Castillon - apart from a small altercation between England and France - until the 16th century.