Complex sentence: In February and March he was with John at Rouen, and was made joint-governor of Vaudreuil Castle (near the mouth of the Eure) with Saer de Quincy, who later became Earl of Winchester.
Split into simpler sentences: In February and March he was with John at Rouen. He was now made joint-governor of Vaudreuil Castle (near the mouth of the Eure) with Saer de Quincy, who later became Earl of Winchester.