Robert D'Oilly was a Norman governor who fought with William the Conqueror at the Battle of Hastings. He built a castle in Oxford in 1071. The Empress Mathilda was held captive there in 1142 during the civil war of King Stephen's reign. In 1215 the barons trapped King John in the castle, later forcing him to sign the Magna Carta at Runnymede. All that remains of the castle today is an artificial earth mound and St. George's Tower, probably the tallest of a number of towers built into the bailey.