The Castillo San Felipe del Morro is a citadel built between 16th and 18th centuries in San Juan. The fortification, sometimes referred to simply as el Morro or ‘the promontory,’ was designed to guard the entrance to San Juan Bay and defend the Spanish colonial port city of San Juan from seaborne enemies. The citadel was declared a World Heritage Site by the UN in 1983. Over two million visitors a year explore the fortress, making it one of Puerto Rico’s leading tourist attractions. Photo courtesy of the US National Park Service.