castles, William's wife convoked the imprisoned Achaean lords' wives to a parliament. Although Guy I opposed the proposal, the "parliament of dames" consented to it because the Achaean ladies wanted their husbands back. Before releasing William, Michael VIII extracted an oath of fealty from him and took two Achaean ladies, Margaret of Passavant and an unnamed sister of John Chauderon, as hostages to secure William's compliance with their agreement. William returned to Achaea late in 1261. After the Byzantines had taken possession of the three castles, the Tzakones and the Melingoi transferred their loyalty to Michael VIII. Conflicts with the Byzantines Neither William nor Michael VIII believed the peace treaty would be lasting: the Byzantines could use their Moreot bridgeheads for further expansion, while William could hardly acquiesce to the territorial losses. As a consequence of the Byzantine expansion in the Morea, William could rarely offer fiefs to western European knights, which diminished his principality's military power. Yet the Byzantines refrained from launching major invasions against Achaea because the Frankish cavalry were still able to inflict severe defeats on them. Instead, they attacked poorly garrisoned fortresses and seized them with the support of the local Greeks and Slavs. Pope Urban IV released William from the oath he had taken at Constantinople under duress, and William started peace negotiations with the Venetians. In May 1262, William abandoned his claim to rule parts of Negroponte directly in return for the recognition of his suzerainty over the island. The Venetians were to destroy their seaside fort at Chalcis, but their quarter in the city was expanded. The possession of the port of Monemvasia allowed the Byzantines to transfer troops to the Morea. By the end of 1262, they seized Cape Maleas and took control of the nearby Mani Peninsula. Pope Urban IV urged the Catholic bishops and abbots of Frankish Greece to support the Achaeans against the "schismatic" Byzantines on 27 April 1263, but he soon realized he could achieve his principal goal, the unity of Christendom, only through negotiating with Michael VIII. Michael VIII sent a new army to Monemvasia in the summer of 1263. The Byzantines invaded Arcadia and seized small fortresses, but the Franks routed them near William's capital at Andravida. Another Byzantine army marched towards Kalavryta and seized it with the support of the local population. Next year, the Byzantine commander John Kantakouzenos launched a new invasion of Arcadia,