Vexin

Vexin (French pronunciation: ​[vɛksɛ̃]) is a historical county of northwestern France. It covers a verdant plateau on the right bank (north) of the Seine running roughly east to west between Pontoise and Romilly-sur-Andelle (about 20 km from Rouen), and north to south between Auneuil and the Seine near Vernon. The plateau is crossed by the Epte and the Andelle river valleys.

The name Vexin is derived from a name for a Gaulish tribe now known as the Veliocasses that inhabited the area and made Rouen their most important city.

The Norse nobleman Rollo of Normandy (ca. 846–ca. 931), the first ruler of the Viking principality that became Normandy, made several incursions into the western half of the county. He halted his actions when the Carolingian king Charles the Simple abandoned the part of the territory that Rollo occupied under the Treaty of Saint-Clair-sur-Epte in 911. The terms of the treaty established the Duchy of Normandy and fixed its boundary with the Kingdom of France along the river Epte. This divided the county of Vexin into two parts:

During the twelfth century, the county of Vexin was a heavily contested border between the Angevin kings of England and Capetian France. It was of particular importance because of the close proximity to Paris and the location of the route to the coastal cities of Normandy. As a result, Vexin was the site of defensive castle construction, notably at Château Gaillard.

...
Wikipedia