The city's second major fort is one of the largest Spanish-built military installations in the Americas. In its prime, it covered 27 acres with a maze of six interconnected forts protecting a central core with 150ft walls, moats, booby-trapped bridges and tunnels. It has a fascinating museum, military archives, a reproduction of military barracks, a store and stunning Atlantic and city views. Hour-long free guided tours in English roam the tunnels at 10:30am on Saturdays (Sundays in Spanish); first-come, first-served.

　　The fort was constructed to defend Old San Juan against land attacks from the east via Puerta de Tierra. The imaginative design came from the famous Irish mercenary Alejandro O’Reilly and his compatriot Thomas O’Daly (hired by Spain). Construction began in 1634 in response to an attack by the Dutch a decade previously, though the main period of enlargement occurred between 1765 and 1783.

　　Seven acres were lopped off the fort in 1897 to ease congestion in the old town and the following year the Spanish marked Puerto Rico’s entry into the Spanish-American War by firing at the battleship USS Yale from its cannon battery. The fort became a National Historic Site in 1949 and part of the Unesco World Heritage site in 1983.