After the Fourth Crusade broke Constantinople in 1204 and the Frankish lords divided the spoils of the Byzantine Empire, the Peloponnese fell to the Principality of Achaea. By around 1209, Crusader forces had completed their conquest of the peninsula, including Nikli — the ancient Tegea — after a siege. The Barony of Nikli was established as one of the twelve original secular baronies of the principality, the foundational administrative units of Frankish Greece. It came with six knight's fiefs attached. The first baron was a lord known in the Greek and Italian versions of the Chronicle of the Morea only as William, but identified by Angevin records as William de Morlay. The barony passed to his son Hugh, and then, around 1280, to Androuin de Villiers (also recorded as de Villa), who had married Hugh's sister Sachette — a reminder that inheritance in Frankish Greece often traveled through women even when the title itself did not.

The town the Franks called Nikli — and also Amyklai or Amyklion in some sources — was the ancient city of Tegea, once the most powerful city in Arcadia, rival to Mantinea, host of the famous Temple of Athena Alea. Under Frankish rule it was also a bishopric, its see taken over by a Latin bishop after the conquest. That arrangement lasted only until 1222, when the Latin bishopric was abolished and its jurisdiction merged into that of Lacedaemon. Nikli's location on the flat southern Arcadian plain made it an effective assembly point for armies — the same characteristic that had made ancient Tegea strategically important. The Franks understood its value for the same reasons the Spartans once had: it sat at a crossroads, visible and accessible from multiple directions.

The defining moment of the Barony of Nikli is also one of the most unusual episodes in medieval Greek history. In 1259, the Achaean forces were routed at the Battle of Pelagonia, and Prince William II of Villehardouin was among those taken prisoner by the Byzantines under Michael VIII Palaiologos. With most of the principality's military leadership in captivity, the noblewomen who remained convened at Nikli to address the crisis. This gathering — the 'Ladies' Parliament' — deliberated on the ransom negotiations and took administrative decisions in the absence of their husbands. William II was eventually released in 1261 in exchange for ceding several key Peloponnesian fortresses to Byzantium, including Mystras, Maina, and Monemvasia. Those fortresses would become the nucleus of Byzantine recovery in the Peloponnese. The Ladies' Parliament at Nikli was a contingency measure that became, unintentionally, a pivot point in the history of Frankish Greece.

The Barony of Nikli was still in Frankish hands in 1280, but Byzantine pressure was steadily reclaiming the Peloponnese, fortress by fortress and town by town. The Aragonese version of the Chronicle of the Morea records that the Byzantines occupied Nikli in 1296 — but then made a pragmatic decision. The town, sitting exposed on the open plain, was indefensible. Rather than hold it, they preferred to destroy and abandon it. The region remained in a contested state for a few more years, but by 1302 it was definitively in Byzantine hands. The Franks had held Nikli for less than a century. The town that had been continuously inhabited since at least the Archaic period of ancient Greece was reduced to rubble not because anyone particularly wanted to destroy it, but because geography made it impossible to defend. It was not rebuilt. The Archaeological Museum of Tegea, two hundred meters from the ancient temple site, is now what stands in that place.