Here you can see the Contrafossa and the Old Fort. Contrafossa is the canal that separates the Old Fort from the old town, and it was the first line of defense. Today it is used as "parking" place for fishing boats. The old town of Corfu and its 3 forts have been declared World Heritage Centres by the UNESCO. Here's some info from the UNESCO website:
"The Old Town of Corfu, on the Island of Corfu off the western coasts of Albania and Greece, is located in a strategic position at the entrance of the Adriatic Sea, and has its roots in the 8th century BC. The three forts of the town, designed by renowned Venetian engineers, were used for four centuries to defend the maritime trading interests of the Republic of Venice against the Ottoman Empire. In the course of time, the forts were repaired and partly rebuilt several times, more recently under British rule in the 19th century. The mainly neoclassical housing stock of the Old Town is partly from the Venetian period, partly of later construction, notably the 19th century. As a fortified Mediterranean port, Corfu’s urban and port ensemble is notable for its high level of integrity and authenticity.
Outstanding Universal Value
The ensemble of the fortifications and the Old Town of Corfu is located in a strategic location at the entrance to the Adriatic Sea. Historically, its roots go back to the 8th century BC and to the Byzantine period. It has thus been subject to various influences and a mix of different peoples. From the 15th century, Corfu was under Venetian rule for some four centuries, then passing to French, British and Greek governments. At various occasions, it had to defend the Venetian maritime empire against the Ottoman army. Corfu was a well thought of example of fortification engineering, designed by the architect Sanmicheli, and it proved its worth through practical warfare. Corfu has its specific identity, which is reflected in the design of its system of fortification and in its neo-classical building