The Barony of Patras was, from the beginning, one of the largest and most consequential in the Principality of Achaea. With twenty-four knight's fiefs attached to it, it ranked alongside the Barony of Akova as the weightiest secular domain in the principality. The first baron was likely a Provençal knight from the Aleman family — the Treaty of Sapienza between Achaea and Venice, concluded in June 1209, names Arnulf Aleman, while the Chronicle of the Morea, the principal narrative source for this period, names William Aleman. The discrepancy is unresolved; history at this distance is rarely tidy. What is clear is that Patras was also the seat of a Latin Archbishopric, with eight additional knight's fiefs of its own, and that the relationship between the secular baron and the archbishop was, from the start, contentious. The baron at one point evicted the archbishop from his residence and incorporated the cathedral of St. Theodore into Patras Castle.

The shift of power came gradually. By the middle of the thirteenth century — possibly as early as the 1220s, when the first Latin archbishop, Antelm of Cluny, already appears to have held the castle — the barony was moving toward ecclesiastical control. The Aragonese version of the Chronicle of the Morea records that William Aleman II sold the barony's rights to the Archbishop around 1276. Whatever the precise timing, the effect was clear: the Archbishop of Patras now commanded thirty-two knight's fiefs, making him the strongest vassal in the entire Principality. A churchman had become the most powerful feudal magnate in the region, which was precisely the kind of outcome that made secular princes nervous.

The archbishopric's drift toward independence came to a head in 1337. Archbishop William Frangipani, who had governed from 1317 and cultivated close ties with Venice, died that year. The Angevin regent Bertrand of Les Baux, whom Frangipani had consistently opposed, moved quickly to lay siege to the city and force submission. Pope Benedict XII responded by declaring Patras 'land of the Holy Roman Church' and placed the entire Principality under interdict. The regent's mother, Catherine of Valois, had little choice but to concede. From 1337 on, Patras was effectively an independent ecclesiastical domain, its archbishops acknowledging the Prince's suzerainty only for their secular holdings while operating as sovereign rulers of the city itself.

Independence was easier to declare than to maintain. For the remainder of the fourteenth century and into the fifteenth, the Archbishops of Patras maneuvered between competing powers — the declining Principality, the encroaching Ottomans, raiding Albanian groups, and the ever-pragmatic Republic of Venice. In 1408, with Ottoman pressure mounting, the Archbishop invited Venice to administer the barony in exchange for 1,000 ducats a year. The Pope objected, and Venice withdrew in 1413. They tried again in 1418; the Holy See objected again. The dance of great powers around a small but strategically vital city continued until, in 1429 and 1430, Constantine Palaiologos — the man who would become the last Byzantine emperor — reconquered the city for the Despotate of the Morea. Two centuries of Frankish and ecclesiastical rule ended where it had begun: with a Greek sovereign.

The physical legacy of the Frankish barony is most visible in Patras Castle, which sits above the modern city on the ancient acropolis hill. The castle's present form owes much to the Latin period — it was the center of secular power during the barony and later the archbishop's residence after the eviction controversy was resolved. Walking the battlements today, with the Gulf of Corinth spread out below and the mountains of the Peloponnese behind, it is possible to understand why this site attracted, and rewarded, two centuries of careful statecraft. Whoever held Patras controlled the entrance to the Corinthian Gulf, and everyone who mattered in medieval Greece understood that.