Naupaktos Castle rises on the western foot of Mount Rigani, on a hilltop two hundred metres high above sea level, which dominates in the broader area and controls the sea passage to the Corinthian Gulf. The castle is one of the most representative monuments of defence architecture in Greece. It has been built with the shape of an amphitheatre: the higher part includes the hill top and slopes, while two branches extend east- and westwards, following the natural terrain and descending towards the sea, where they join a small fortified port.
The earliest fortification works in Naupaktos go back to Classical Antiquity. The ancient fortification extended further than the medieval one: it included lowlands and coastal areas in the eastern part of the town, and attests to the great size of the ancient city. It extended to the harbour, and was protected with square towers. Sections of the ancient fortification were integrated to the medieval defence wall: in the western branch of the descending walls and in the eastern branch, above the Faltsoporti Gate. On the citadel, fragments of the ancient towers, built of well dressed stone blocks in pseudo-ashlar, trapezoid and polygonal masonries, form the base of the Byzantine towers. Numerous single ancient blocks have been re-used in Byzantine and Venetian masonries.
The earliest medieval fortification was erected in the Middle Byzantine period, during the 10th or 11th centuries, and is connected with the upgrade of Naupaktos as capital of the thema of Nikopolis. The facts that the hilltop housed a pre-existing ancient fort, had natural water supply and was naturally defended were the reasons why the medieval fort was erected on it. The Byzantine fort was limited to the top of the hill, where the modern archaeological site extends, while the lower town and the suburbs were not walled. It circumscribed the hilltop from all sides, following the natural terrain, and was strengthened with high rectangular and semi-cylindrical towers. Both the curtain wall and the towers were built with ancient stone blocks, rubble masonry, mortar and visible tiles. They had small openings and rectangular battlements. During this period, the citadel did not only have a prominent strategic role, but was also the administrative and religious centre of the town.
The capitulation of Naupaktos in 1407 by the Venetians, and the need to convert it to a defensible base at the entrance of the Corinthian Gulf, introduced the design of a new defence wall which included the main part of the town and the military harbour. It is during the first Venetian Period (1407-1499) that the fortified town obtains its current form. The Venetian walls were constructed mainly during the 15th century and bear characteristics of the early Venetian defence architecture.
Construction works by the Ottoman Turks during the years 1499-1687 and 1699-1829 were the structural reinforcement of the walls and their adaptation to the new defence needs against fire weapons.