Dubrovnik City Walls Fortresses are historical evidence found in the old city walls. These walls contain many fortresses and castles, each of which dates back to a specific era. Among these castles is the lack of St. John. It is a huge castle located on the southeast side of the port of the old city and was built to protect the city from dangers. It is a wonderful castle and the view is amazing so don't hesitate to visit it when you go to Dubrovnik.