Inhabited since prehistoric times, Acronauplia was fortified by at least the late 4th century BCE, when Nafplio served as the harbour of the neighbouring powerful city of Argos. In the following centuries, the small settlement of Nafplio remained in Argos’ shadow, functioning as its seaport. After the 10th century, Nafplio began to gain in prominence, becoming a trading post for the Venetians and gradually obtaining the same significance as Argos. The 12th century was a period of prosperity, as shown by the establishment of a new administrative unit, the Frontier of Corinth, Nafplio and Argos, which has been associated, among other things, with fortification works at the three castles. In 1180, Byzantine Emperor Manuel Komnenos appointed Theodoros Sgouros as lord of Nafplio. Around 1200, he was succeeded by his son Leon who expanded his power from Nafplio a far north as Thessaly. Leon’s brief rule ended when the knights of the Fourth Crusade occupied Acronauplia from 1210 to 1212.

Geoffrey of Villehardouin, the conquering Frankish ruler of Achaia, ceded Nafplio to Otto de la Roche, the duke of Athens. The heirs of the latter, initially from the de la Roche family and then from the houses of Brienne and later Enghien, retained sovereign rights over the castle of Nafplio until 1389, when they sold them to the Republic of Venice.

The growing threat of the Ottomans, who occupied the Venetian castles of Argos in 1467 and Chalkida in 1470, made it imperative to adapt Nafplio’s defences to the new firearms technology. The now dominant Venetians carried out extensive works in the late 15th century to modernise and strengthen the defence of the city, which they called Napoli di Romania.

At the same time, population growth prompted the expansion of the city into the marshy area north of the peninsula, now the flat part of Nafplio’s Old Town. The new extension, the “Lower Town”, was protected by a coastal defensive wall constructed during the same period. It was built on artificial alluvium and wooden pile foundations, techniques also used in Venice. Gradually, the administrative centre and officials’ residences were transferred from Acronauplia to the new part of the city.

In 1540, after a three-year siege, Nafplio surrendered to the Ottomans, who designated it the capital of the Sanjak of the Peloponnese. During this period, they constructed large public and private buildings, mosques, fountains and baths. However, the Ottomans made no new additions to the city’s fortification, undertaking only minor interventions and changes. Acronauplia became known as Iç Kale (“inner castle” or “citadel”), in contrast to the Lower Town.

In 1686, during the Sixth Venetian-Οttoman War and the campaign to recapture the Peloponnese, the Venetians, under the command of Francesco Morosini, besieged and captured the city, causing extensive damage to both the walls of Acronauplia and the Lower Town, as well as to buildings and districts. Nafplio became the seat of the Venetian Kingdom of the Morea and an extensive programme to reinforce its fortifications was undertaken in anticipation of future Ottoman attacks and sieges.

Immediately after capturing the city, the Venetians prohibited habitation within the castle, parts of which were levelled to make way for powerful artillery batteries, and its function became exclusively military. A large barracks and a gunpowder storeroom were built under the direction of Proveditore Antonio Grimani on the site where a hotel now stands. In 1711, construction began on the Palamidi Fortress on the hill dominating the east of the city, marking the decline in the military importance of Acronauplia for the defence of Nafplio.

The second Venetian occupation, a period of prosperity for the city, was abruptly interrupted in 1715, when the Ottomans recaptured Nafplio after a siege lasting only two weeks. In the years that followed, the city’s administrative structure declined, particularly after the Ottomans relocated their headquarters to Tripolitsa (modern Tripoli), contributing to its economic decline. Acronauplia Castle remained in military use and housed an Ottoman garrison. In 1822, following the capture of Palamidi by Greek revolutionaries, the city and its castle were handed over to Theodoros Kolokotronis.

During the Greek Revolution, the newly established Greek state designated Nafplio as its administrative centre and capital. The state’s first governor, Ioannis Kapodistrias, oversaw the reconstruction of the castle’s ruined Grimani Barracks, which became known as Kapodistrias Barracks, and established a military hospital with a church dedicated to the Holy Unmercenaries. Acronauplia was also cleared of piles of ruins.

In 1834, Athens was declared the capital of the Greek state, while Nafplio remained a military centre. In 1884, a military prison was established in Kapodistrias Barracks, which was turned into a civilian prison in 1937. During World War II, the highest point of the Acronauplia rock was levelled to accommodate anti-aircraft emplacements. In 1960, the castle was designated as a Tourism Public Property and in 1961, the Xenia hotel, managed by the Greek National Tourism Organisation, opened on a significant part of Tori Castle. Acronauplia’s prisons were abandoned in 1966 and demolished in 1970. In their place, construction began in 1971 of the Xenia Pallas hotel complex, which continues to operate on the site.