treaty, Baldwin II ceded all his suzerain rights over Achaea to Charles and his successors in return for Charles' promise to assist him to recover Constantinople within six or seven years. As Charles's loyal vassal, William led 400 knights to fight against Charles's opponent, the young Hohenstaufen prince Conradin in the Battle of Tagliacozzo on 23 August 1268. On his return to Achaea in January 1269, William captured Valona to establish a secure bridgehead for Charles's troops on the eastern shore of the Adriatic Sea. The Achaean barons formally recognized the Treaty of Viterbo in the presence of Charles's envoys in 1270. Next year, Charles appointed the Marshal of Sicily, Dreux of Beaumont, to represent him as captain general in Achaea. At the same time, the adventurer Licario had seized a sturdy fortress near Karystos in Euboea. He made an alliance with the Byzantines, and with their support, he undertook to conquer the entire island. William and Dreux launched a joint military campaign against Licario and recaptured the fortress La Cuppa at Avlonari from him. The Byzantines launched two major invasions against Achaea between 1270 and 1275, but William was able to repel them with the support of Angevin troops. Walter of Rosières, who held the Barony of Akova in Achaea, died childless and his heir, Margaret of Passova was still held as hostage for William in Constantinople. William claimed the barony escheated to the crown because she failed to claim it personally within the customary period of a year and a day. After her return from Constantinople, Margaret revived her claim to Akova and took the influential aristocrat John of Saint Omer as her husband to secure his support. Saint Omer convinced William to compensate Margaret, and he restored one-third of the barony to her in 1276. As William's son-in-law Philip died childless early in 1277, it was evident that Charles or Charles's offspring would inherit Achaea from William on his death. He died on 1 May 1278. With his death, the male line of the Villehardouin family became extinct. Charles succeeded him without opposition, but he allegedly never visited Achaea. Galeran of Ivry represented Charles in the principality as bailli and vicar-general. Legacy Historian Kenneth M. Setton describes William as "the most lordly of the princes of Achaea", emphasizing the strength and prestige of Achaea reached its zenith during William's rule around 1250. The author Nicolas Cheetham writes