Champlitte and Geoffrey of Villehardouin marched south through mainland Greece and conquered the Peloponnese against sporadic local Greek resistance. The Franks then founded the Principality of Achaea, nominally a vassal of the Latin Empire, while the Venetians occupied several strategically important ports around the coast such as Navarino and Coron, which they retained into the 15th century. The Franks popularized the name Morea for the peninsula, which first appears as the name of a small bishopric in Elis during the 10th century. Its etymology is disputed, but it is most commonly held to be derived from the mulberry tree (morea), whose leaves are similar in shape to the peninsula. In 1208, William I founded a commission at Andravida consisted of Latin bishops, two bannerets and five Greek magnates and chaired by himself, to assess the land and divide it, according to Latin practice, in fiefs. The result was divide the country into twelve baronies, mostly centred around a newly constructed castle—a testament to the fact that the Franks were a military elite amidst a potentially hostile Greek population. The twelve temporal barons were joined by seven ecclesiastic lords, headed by the Latin Archbishop of Patras. Each of the latter was granted a number of estates as knightly fiefs, with the Archbishop receiving eight, the other bishops four each, and likewise four granted to each of the military orders: the Templars, Hospitallers and the Teutonic Knights. Shortly after 1260, a thirteenth barony, that of Arcadia (modern Kyparissia) was established, which was also a personal fief of the Villehardouins. The barons retained considerable powers and privileges, so that the Prince was not an absolute sovereign but rather a "first among equals" among them. Thus they had the right to construct a castle without the Prince's permission, or to decree capital punishment. Since Salic Law was not adopted in Achaea, women could also inherit the fiefs. Despotate of Morea and Ottoman incursions Frankish supremacy in the peninsula, however, received a critical blow after the Battle of Pelagonia, when William II of Villehardouin was forced to cede the newly constructed fortress and palace at Mystras near ancient Sparta to a resurgent Byzantium. At this point, the emperor concluded an agreement with the captive prince: William and his men would be set free in exchange for an oath of fealty, and for the cession of Monemvasia, Grand Magne, and Mystras. The handover was effected in