Mowbray did not attend and ignored further summonses, so that William finally led an army against him. Mowbray shut himself up in his stronghold, Bamburgh Castle. William laid siege to Bamburgh and built a temporary siege castle alongside it, known as "Malvoisin", or "evil neighbour". For some reason, during the siege, Mowbray left the castle with a small force of knights and was pursued by his besiegers, being forced to take refuge in Tynemouth. After a siege of six days he was wounded in the leg, captured and was taken back to Bamburgh where his wife was still resisting the besiegers. She finally surrendered the castle after the besiegers threatened to blind her husband. As a result of his part in the rebellion Mowbray forfeited his estates and was imprisoned for life, initially at Windsor Castle. He spent many years in prisons, "growing old without offspring", according to the chronicler, Florence of Worcester, and then was allowed to become a monk at St Albans Abbey, according to another chronicler Orderic Vitalis. There is some doubt about the date of his death. On one hand it was claimed that he spent thirty years in prison, giving his date of death about 1125. However, William Dugdale claimed that Mowbray became a monk and died in 1106. Mowbray’s fellow conspirators, William of Eu and William of Aldrie, received harsher punishment, William of Eu being castrated and blinded, and William of Aldrie being condemned to death. Orderic Vitalis gives the following description of Robert de Mowbray: "Powerful, rich, bold, fierce in war, haughty, he despised his equals and, swollen with vanity, disdained to obey his superiors. He was of great stature, strong, swarthy and hairy. Daring and crafty, stern and grim, he was given more to meditation than speech, and in conversation scarce ever smiled". Mowbray's wife, Matilda, was granted an annulment of her marriage by Pope Paschal II and sometime after 1107, she became the wife of Robert's brother Nigel d'Aubigny, who was also granted the lands in Montbray forfeited by her former husband. The couple remained childless and in 1118 d’Aubigny divorced Matilda and married Gundred de Gournay, daughter of Gerard de Gournay and Edith de Warenne.