William's next efforts were against Guy of Burgundy, who retreated to his castle at Brionne, which William besieged. After a long effort, the duke succeeded in exiling Guy in 1050. To address the growing power of the Count of Anjou, Geoffrey Martel, William joined with King Henry in a campaign against him, the last known cooperation between the two. They succeeded in capturing an Angevin fortress, but accomplished little else. Geoffrey attempted to expand his authority into the county of Maine, especially after the death of Hugh IV of Maine in 1051. Central to the control of Maine were the holdings of the Bellême family, who held Bellême on the border of Maine and Normandy, as well as the fortresses at Alençon and Domfront. Bellême's overlord was the king of France, but Domfort was under the overlordship of Geoffrey Martel and Duke William was Alençon's overlord. The Bellême family, whose lands were quite strategically placed between their three different overlords, were able to play each of them against the other and secure virtual independence for themselves.