Saint John's Fortress, Limestone fortress in Old Harbor, Croatia.

Saint John's Fortress is a semicircular stone fortress on the southeast side of Dubrovnik's old harbor, formed when two earlier forts were joined by towers in 1522 and consolidated by 1557. Its lower level holds an aquarium of Adriatic fish, while the upper floor houses the Dubrovnik Maritime Museum. The terrace served as a summer stage for the Dubrovnik Summer Festival.

Construction began in 1346 and continued through multiple phases until 1557, when two earlier forts merged into the present structure. This consolidation significantly strengthened the city's defense against threats from the sea.

The Maritime Museum inside displays seafaring history through ship models and tools used by medieval traders. The collections show how this harbor supported the city's role as a major Mediterranean trading center.

Visitors should know that the Maritime Museum, aquarium, and art spaces require separate entrance tickets and operate independently. Plan several hours if you want to see everything inside.

The terrace of the fortress serves as a summer stage for the Dubrovnik Summer Festival, and a cannon taken from the sailing ship Triton was once mounted there. The ground floor holds an aquarium of Adriatic fish, while the upper floor houses the Maritime Museum.