The Principality of Achaea was the Frankish territory in the Peloponnese ruled by the princes of Achaia from 1205 to 1430. In the aftermath of the Fourth Crusade and the Latin conquest of Constantinople in 1204, two Frankish Crusaders, William I of Champlitte and Geoffrey I Villehardouin, seized control of virtually the entire Byzantine Peloponnese and became the first two princes of Achaia. The Frankish principality reached the peak of its power under William II Villehardouin, who constructed fortresses at Mystras, Maina, and Monemvasia. After William II was captured by the Byzantine at the Battle of Pelagonia in 1259, however, and forced to cede the castles to Emperor Michael VIII Palaiologos, the Byzantine regained a foothold in the Morea. During their remaining 170 years of empire, the Byzantine gradually reconquered the Peloponnese, until finally bringing an end to the principality only 30 years before the Despotate of Morea fell, in turn, to the Ottomans. Both the Western and Greek versions of the Chronicle of the Morea are important sources for the first century of the principality. 
The French conquerors imposed a feudal system upon their Greek territory. The prince of Achaia was nominally a vassal of the Latin emperor of Constantinople. In reality, however, he controlled more territory than his suzerain and was supported by a larger army. His chief residence was Andravida. The prince had the right to mint coins, which were produced at the active port of Clarenza. The prince's authority was limited by the power of his barons, who were considered his peers; they had private armies and built (or restored) castles throughout the principality at such sites as Old Navarino, Kyparissia, and Karytaina. After Achaia became a dependency of the kingdom of Sicily in 1267 and after the death of William II in 1278, many princes of Achaia held the title only nominally and rarely, if ever, visited the Peloponnese. The French settlers were always outnumbered by their Greek subjects, who sometimes preferred the tolerant French rule to Byzantine administration, but were reluctant to relinquish their Orthodoxy. A Latin ecclesiastical hierarchy was established with the principal archbishop at Patras, subordinate to the Latin patriarch of Constantinople; Greek priests came under the jurisdiction of Latin bishops.