Gardiki Castle (Greek: Κάστρο Γαρδικίου) is a 13th-century Byzantine castle on the southwestern coast of Corfu and the only surviving ancient fortress on the southern part of the island. It was built by a ruler of the Despotate of Epirus, and was one of three castles which defended the island before the Venetian era (1401–1797). The three castles formed a defensive triangle, with Gardiki guarding the island’s south, Kassiopi Castle the northeast and Angelokastro the northwest.
Remains from the upper Paleolithic era, dating from 20,000 B.C., when Corfu was still united to the mainland region of Epirus, were found on the site of the castle at the rock shelter of Grava Gardikiou, including hunter-gatherer stone tools and animal bones, which have since been removed and are exhibited at the Archaeological Museum of Corfu.
The location of Gardiki at the narrow southwest flank of Corfu provided protection to the fields and the southern lowlands of Corfu and in combination with Kassiopi Castle on the northeastern coast of the island and Byzantine Angelokastro protecting the northwestern shore of Corfu, formed a triangular line of defence which protected Corfu during the pre-Venetian era.
The walls of Gardiki Castle form an octagon and the structure features eight strong towers decorated by rows of tiles. There are elements from an ancient structure which have been incorporated into the construction of the castle. The ancient structure was probably a fountain-house.
The eight towers are square in shape and the outline of the octagonal structure is almost elliptical. At the top of the southern tower there are traces of a chapel with remnants of religious frescoes of portraits of saints. Although in a state of disrepair, the towers still retain their full height. The castle entrance is preserved but the interior is in a state of ruin.
Gardiki Castle is considered one of the most imposing architectural remains in the Ionian Islands, along with Angelokastro, Kassiopi Castle and the two Venetian Fortresses of Corfu City, the Citadel and the New Fort.