Geoffrey I of Villehardouin (French: Geoffroi I de Villehardouin) (c. 1169 – c. 1229) was a French knight from the County of Champagne who joined the Fourth Crusade. He participated in the conquest of the Peloponnese and became the second prince of Achaea (1209/1210–c. 1229).
Under his reign, the Principality of Achaea became the direct vassal of the Latin Empire of Constantinople. He extended the borders of his principality.
William of Villehardouin (French: Guillaume de Villehardouin; Kalamata, c. 1211 – 1 May 1278) was the fourth prince of Achaea in Frankish Greece, from 1246 to 1278. The younger son of Prince Geoffrey I, he held the Barony of Kalamata in fief during the reign of his elder brother Geoffrey II. William ruled Achaea as regent for his brother during Geoffrey's military campaigns against the Greeks of Nicaea, who were the principal enemies of his overlord, the Latin Emperor of Constantinople Baldwin II. William succeeded his childless brother in the summer of 1246. Conflicts between Nicaea and Epirus enabled him to complete the conquest of the Morea in about three years. He captured Monemvasia and built three new fortresses, forcing two previously autonomous tribes, the Tzakones and Melingoi, into submission. He participated in the unsuccessful Egyptian crusade of Louis IX of France, who rewarded him with the right to issue currency in the style of French royal coins.
In the early 1250s, William was the most powerful ruler of Frankish Greece. Most neighboring Frankish rulers acknowledged his suzerainty. In 1255, he laid claim to the northern terziere, or third, of the Lordship of Negroponte on the island of Euboea. Although the two other rulers of Negroponte were his vassals, they rejected his claim. They gained the support of Venice, Guy I de la Roche, Lord of Athens, and other Frankish rulers. The conflict developed into a war of succession that caused much destruction in Euboea and mainland Greece. After William's victory in Attica in May 1258, Guy and his allies surrendered. Guy was tried for his disloyalty but was allowed to keep his Achaean fiefs.
During the late Middle Ages, the two cities of Argos (Greek: Άργος, French: Argues) and Nauplia (modern Nafplio, Ναύπλιο; in the Middle Ages Ἀνάπλι, in French Naples de Romanie) formed a lordship within the Frankish-ruled Morea in southern Greece.
Following their conquest in 1211–1212, the cities were granted as a fief to Otto de la Roche, duke of Athens, by Geoffrey I of Villehardouin, prince of Achaea. The lordship remained in the possession of the de la Roche and the Brienne dukes of Athens even after the conquest of the Duchy of Athens by the Catalan Company in 1311, and the Brienne line continued to be recognized as dukes of Athens there. Walter VI of Brienne was largely an absentee lord, spending most of his life in his European domains, except for a failed attempt in 1331 to recover Athens from the Catalans. After his death in 1356 the lordship was inherited by his sixth son, Guy of Enghien. Guy took up residence in Greece, and in 1370–1371 Guy and his brothers launched another, also failed, invasion of the Catalan domains. When Guy died in 1376, the lordship then passed to his daughter Maria of Enghien and her Venetian husband Pietro Cornaro, who would also reside there until his death in 1388. The lordship became a de facto Venetian dependency during this period, and shortly after his death, Maria sold the two cities to Venice, where she retired. Before Venice could take possession, Argos was seized by the Despot Theodore I Palaiologos, while his ally, Nerio I Acciaioli seized Nauplia. The latter city was soon captured by Venice, but Argos remained in Byzantine hands until 1394, when it too was handed over to Venice.