The castle of Koroni is one of the most important monuments of the Venetian period. Built in an eminent geographical site, it was the successor of the ancient city of Asine. The castle was founded in the byzantine period. In 1209 the Venetians conquered the existing byzantine fort, they re-built it and transformed Koroni into a powerful commercial port with international reach. Aiming to adapt to the needs of the new warfare technology, broughtτ about by the adoption of gun powder, the walls were constantly modified and reinforced by the Venetians and later by the Ottomans (1500-1685). Enhancements were also made during the short-lived second Venetocracy (1685-1715). In 1828 the castle was surrendered to the French expeditionary corps of general Maison and was subsequently handed over to Nikitas Stamatelopoulos (Nikitaras), first garrison master of the liberated Koroni.

The castle comprises two baileys: a smaller one to the west, which was the original byzantine core and a broader, later, one to the east. Nine orthogonal towers and five bastions were reinforcing the walls. The main gate opens to the north, while a smaller one at the eastern side was connecting the castle to a narrow strip of land called Livadeia or Livadi. Inside the castle the remains are preserved of the episcopal church of Agia Sofia, a three-aisle basilica of the 7th -8th century, with a second phase in the 11th -12th century, the ruins of an ottoman bath, cisterns and the cemetery church of Agios Charalambos that was built as the catholic church of St. Rocco and was later turned into a mosque. The western part of the castle is now occupied by the monastery of Timios Prodromos, founded in the early 20th century.