 The St. John Fortress (Croatian: Sveti Ivan), often called Mulo Tower, is a complex monumental building on the southeastern side of the old city port, controlling and protecting its entrance. The first fort was built in the mid 14th century, but it was modified on several occasions in the course of the 15th and 16th centuries, which can be seen in the triptych made by the painter Nikola Božidarević in the Dominican monastery. The painting shows Saint Blaise, the patron saint of Dubrovnik. Dominant in the port ambiance, the St. John Fortress prevented access of pirates and other enemy ships. Always cautious at the first sign of danger, the inhabitants of Dubrovnik used to close the entry into the port with heavy chains stretched between the St. John Fortress and the Kase jetty, and they also used to wall up all the port entries to the Great Arsenal.
