In 911 the Treaty of Saint-Clair-sur-Epte established the river as the historical boundary between Normandy and Île-de-France. This divided the traditional county of the Vexin into two parts. The Norman Vexin became a part of Normandy while the French Vexin remained a part of the Île-de-France. A series of castles were built on both banks of the Epte to guard this frontier. 