San Juan’s second major fort is Fuerte San Cristóbal, one of the largest military installations the Spanish built in the Americas. In its prime, San Cristóbal covered 27 acres with a maze of six interconnected forts protecting a central core with 150ft walls, moats, booby-trapped bridges and tunnels. The fort has a fascinating museum, a store, military archives, a reproduction of military barracks, and stunning Atlantic and city views.
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.