Complex sentence: William of Talou had built a strong castle at Arques, and from it (in 1053) he defied his nephew the youthful duke of Normandy: as "family", the comital house of Ponthieu supported the rebellion.
Split into simpler sentences: William of Talou had built a strong castle at Arques, and from it (in 1053) he defied his nephew the youthful Duke of Normandy. As "family", the comital house of Ponthieu supported the rebellion.