In his last enterprise, Guiscard mounted an attack on the Byzantine Empire, taking up the cause of Raiktor, a monk pretending to be Michael VII, who had been deposed in 1078 and to whose son Guiscard's daughter had been betrothed. He sailed with 16,000 men, including 1,300 Norman knights, against the empire in May 1081. He defeated Emperor Alexius I Comnenus at the Battle of Dyrrhachium in October 1081, and by February 1082 he had occupied Corfu and Durazzo. He was recalled to the aid of Gregory VII, however, who was besieged in Castel Sant'Angelo by Henry IV, in June 1083. Also in 1083, Guiscard destroyed the town of Cannae, leaving only the cathedral and bishop's residence.