Legend has it that the city of Nafplio was founded by Nauplius, son of Poseidon and Amymone, daughter of King Danaus of Argos. His son was the hero Palamedes, who gave his name to the high rocky mound to the southeast. The city was founded on a small peninsula 85m high, 900m long and roughly 400m wide. In modern times it was named Akronafplia in replacement of its Turkish name (Iç Kale). It was accessible only from the north, via the neck of Arvanitia, a spit of land created by silting. From the start, the city served as the port of Argos and always lay in its shadow and under its influence.

Nothing is known of Nafplio’s history in Early Byzantine times. However, the city was probably populated by people from the surrounding areas on account of Slavic raids in the late 6^th century. In the early 10^th century the people of Nafplio laid claim to the reliquary of Bishop Peter from the city of Argos, which still honours him as its patron saint. Later in the same century Saint Nicon the Repenter passed through Nafplio while on his mission to the Peloponnese. In 1032, Nicephorus Karantinos, patrician and general of Nafplio, defeated the Arabs in a sea battle. By the 12th century the city had emerged as a place with an important role: testimony from the Arab geographer al-Idrisi is supplemented by references to the region’s monastic centres in the constituent documents of Areia Monastery, compiled by Bishop Leo of Argos and Nafplio, and by the fact that the diocese was upgraded to an archdiocese in 1189. At the same time, Leo Sgouros, son of a powerful local family, became ruler of Argos and Corinth, before besieging Athens and conquering Larissa in Thessaly. The Franks of the Fourth Crusade drove him to suicide at the end of the siege of Acrocorinth, and then laid siege to Nafplio. Five years later the city surrendered to Geoffroi de Villehardouin, who ceded it to Otto de la Roche, Duke of Athens, in 1212.

In the Byzantine period the late classical citadel enclosing the west part of the Akronafplia peninsula was reinforced on the west, north and east sides. To the south, the mound was so steep and inaccessible that it had no need of fortification. Only a few structural remains of the Byzantine city inside the walls have been brought to light. The small three-nave church excavated in the south part of the castle was probably dedicated to Agios Theodoros the Commander, as indicated by a steatite representation of the saint, which is a unique example of a Byzantine miniature. To the north, the sea reached the edge of the rock and it is assumed that on this side, which was partially hidden and sheltered from the winds, a small seaport facility would have been built. Over time a number of workshops, shops and humble dwellings were built there.

In the Frankish period (1212-1389), an intermediate wall was built to divide the castle into two unequal parts, a larger one to the west and a smaller one to the east, towards the neck of Arvanitia. The west section, known in sources as the Romeiko (Greek) castle was virtually isolated, since communication with the outside was only possible via the east castle, where the Franks founded their military and administrative buildings. The east gate, the main entrance to the Frankish castle, was protected by two round towers on the corners. After the dramatic events of the 14^th century between the Franks and the Byzantines in the Peloponnese, in 1389 Marie d'Enghien, last heir of Nafplio, sold the castle to the Serene Republic.

In the first period of Venetian rule, development of the port and an economic boom led to population growth. This spiralled in 1500, when the Serene Republic ruled that residents who had remained in the city for over seven years could obtain citizenship. A drainage program involving wooden poles and beach embankments was carried out in order to meet the new demands, resulting in reclaimed land and a pier. Thus Akronafplia became a citadel, and the new land to the north the lower city. It then became necessary to carry out infrastructure projects and build new fortifications. On the one hand, the city's water supply was secured using a duct to bring drinking water from the ancient Kanathos spring, next to Areia Monastery, while brick sewers carried waste water to the sea. On the other, the lower city was enclosed by strong walls that hugged the new coastline. In 1470 the Venetians built the Castello di Toro, a third castle for greater protection, to the east of the two earlier ones on the citadel. Designed by Antonio Gambello, it was built according to the latest specifications (with a sloping scarp, a horizontal cornice and circular low bastions), so as withstand the new siege techniques imposed by the use of gunpowder and cannon. In the same period fortifications were built on Bourtzi, the small island off the city’s seafront.

After the end of the Venetian-Turkish War, Nafplio and Monemvasia were ceded to the Ottomans. In the first Ottoman period (1540-1686), Nafplio was divided into five districts, with the Ottoman population residing mainly in Akronafplia and its foothills and the Greeks in the rest of the lower city. The homes depicted in old engravings are all built up the hillside, with projecting wooden sun rooms on the upper floor. Fountains, the seraglio of the Pasha of Morea (Mora Valesi) and the mosques in the city market (e.g. the present day Trianon) were all built in this period.

In 1686 the city was retaken by the Venetians, who initiated a major building and fortification project that lasted until 1714. Commander in chief Francesco Morosini turned Akronafplia into a military citadel. Work on the Palamidi fortifications began in 1690, when the walls of Akronafplia and the lower city were completed and reinforced with ramparts, bastions (Grimani Bastion), the imposing Land Gate, which was protected by a moat, and the great cistern in the north-west corner of the lower city. To facilitate communication with the lower city, Provedditor Agostino Sagredo built a new gate on the north side of the citadel. Some of the most important buildings were erected at this time, such as the Armoury (1713), which now houses the Archaeological Museum.

Although Nafplio was designed to be impregnable, in 1715 it was seized by the Ottomans, who turned it into a typical Muslim city, complete with baroque buildings, mosques and seminaries, fountains and baths. The city fell to Greek revolutionaries on 30 November 1822, and became the first capital of the newly established Greek Kingdom.