William de Mohun was a knight of great valour. He improved the buildingsof his predecessor at Dunster, and was a great benefactor to the priorythere, as well as to that at Taunton, which was founded by WilliamGiffard, Bishop of Winchester. William sided with the Empress Maud in thefight against King Stephen, and in 1138 Dunster Castle was beseiged byStephen, who, however, found it too strong to take. He married Agnes,daughter of Walter de Gaunt or Gant, who was grandson of Baldin, 6thCount of Flanders, who married Matilda, sister of William the Conqueror,thus making Walter de Gant 1st cousin to Kings William Rufus and Henry I.