It was one of the three Byzantine castles that safeguarded the island before the Venetian period (1386–1797). These castles formed a defensive triangle, with Gardiki defending the south of the island, Kassiopi the northeast, and Angelokastro the northwest.

Its location on the northeast coast, overlooking the Corfu Channel which separates the island from the mainland, provided the castle with a strategic advantage and high military significance.

The Castle of Kassiopi is considered one of the most significant architectural remains in the Ionian Islands, along with Angelokastro, Gardiki Castle, and the two Venetian Fortresses of Corfu Town, the Old Fortress, and the New Fortress.