Guy was defeated by William and his overlord, King Henry I of France, at the Battle of Val-ès-Dunes. He retreated to his castle at Brionne, where the Duke besieged him there for three years before finally subduing him in 1049. William of Poitiers wrote that the Duke subsequently allowed Guy to remain at his court; William of Jumièges describes this as house arrest. As a punishment for his rebellion, Guy lost his castles.