Fort San Felipe del Morro © Farragutful

Built in 1540, the mighty six-level fortress of San Felipe del Morro towers 140 feet (43m) above the sea on San Juan Bay, its 18-foot thick (5m) walls having proved a worthy defence against many an invasion. One of the largest fortifications in the Caribbean, it is a maze of tunnels, dungeons, barracks, lookouts and ramps, offering spectacular views from atop its ramparts. In Old San Juan, stands El Morro's partner in defending the city, Castillo San Cristobal, built in the 17th century to a confusing and intricate modular design.

Website: www.nps.gov

Opening times: 9am - 6pm