The old fort in Corfu was built by the Venetians.
The fortress covers the promontory which initially contained the old town of Corfu that had emerged during Byzantine times.
As part of their defensive plans the Venetians separated the promontory from the rest of the city of Corfu by creating the Contrafossa, a moat which is a sea channel connecting the Gulf of Kerkyra to the North with the Bay of Garitsa to the South, converting the citadel into an artificial island. The fort successfully repulsed all three major Ottoman sieges: the great siege of 1537, the siege of 1571 and the second great siege of Corfu in 1716.
More pictures are here