exempted them from feudal obligations. William's conquest of Monemvasia forced the Tzakones of nearby Mount Parnon into submission. To secure his gains, William ordered the construction of new fortresses. First, in late 1248 and early 1249, he personally directed the construction of Mistra near the Mount Taygetus; then the castle of Grand Magne was built on the Laconian Gulf. A third castle, Beaufort, was constructed on the Messenian Gulf. These castles secured the Frankish control of the Mount Taygetus, forcing the local Slavic tribe of the Melingoi to acknowledge William as their ruler in return for the confirmation of their liberty. Hugh IV, Duke of Burgundy spent the winter of 1248–49 in Achaea. He was on the way to Cyprus where the participants of Louis IX of France's crusade against Egypt were assembling. William decided to join the crusade and mustered 400 knights and armed a fleet of 24 ships before he left for Cyprus together with Hugh in May 1249. On the way, he sent 100 knights to Rhodes, an island that the Genoese had recently conquered from the Nicaeans, to strengthen its defence. From Cyprus, William accompanied Louis to Egypt and remained with him until the end of the abortive military campaign. As a reward, Louis granted him the right to mint coins in the style of the French . William returned to Achaea in May 1250. During the following five peaceful years, Achaea was the dominant power of Frankish Greece as most of the lesser Frankish rulers acknowledged William as their overlord. War of the Euboeote Succession Carintana dalle Carceri was one ruler of Negroponte, sharing Oreus and the island's northern triarchy, or third, with Grapella of Verona. When she died in 1255, William wanted to seize her lordship, but Grapella laid claim to her inheritance. As the triarchs owed allegiance to both Achaea and Venice, Grapella could cite a 1216 ruling by the Venetian , or governor, of Negroponte, that stated a co-ruler of a triarchy was entitled to re-unite it if his or her partner died without issue. The lords of the island's two other triarchies, Guglielmo I da Verona and Narzotto dalle Carceri, supported Grapella's claim. The Venetian chronicler Marino Sanudo writes that the conflict developed into a war after William had Guglielmo and Narzotto imprisoned, because their wives convinced the Venetian Paolo Gradenigo to intervene and he seized the island's capital, Chalcis. If