It is located on the south-eastern corner of the city and was named after the nearby medieval Church of St. John. Throughout history it was also called the Tower by the Pier and the Harbour Chain Tower. It served as the main defence of the city harbour from attacks by sea.
Construction of the fortress began in 1346, and over the years it developed from a late medieval tower into a massive, irregularly shaped Renaissance fortress, whose final scope and appearance were designed by Paskoje Miličević in 1557.
Apart from the stone cornices, the only decoration of the fortress is a niche with a statue of St. Blaise from 1559, the work of the French master Jacob de Spinis from Orléans and the stone mason Vicko Lujov from Korčula.
The ground floor of the fortress houses the Aquarium, while the first and second floors contain the Maritime Museum. One of the entrances to the city walls is also located there.
