This was the main defence for the city port and one of the most important defence fortresses of the city. The first tower in this location was erected in the middle of the 14th century and today is a constituent part of this fortress. Later, over several decades, the tower was expanded, fortified and adapted. It received its current semicircular form and the monumental appearance of the entire structure in the 16th century. Standing guard on the other side of the harbour is the tower of St. Luke, one of the oldest preserved city towers, constructed in the 13th century. The city port was closed off at night, by a chain which stretched from the Tower of St. Luke to the Mula Tower, today the Fort of St. John. At the end of the 15th century, the port jetty, called Kase, was built by the renowned Dubrovnik engineer Paskoje Milicevic. With the construction of the Kase jetty, the port was doubly protected: from high waves and adverse sea conditions while also reinforcing the defence system in the port to protect from the eternal threat of attack from the sea.