The fortress in San Juan, Puerto Rico, that is a UNESCO World Heritage Site is El Morro, formally Castillo San Felipe del Morro.
El Morro occupies a rocky headland at the entrance to San Juan Bay in Old San Juan. Spain commissioned its first fortified structure in 1539, and the complex expanded over more than two centuries into the massive citadel visible today. Its name honors King Philip II of Spain; “El Morro” means “the promontory,” describing its commanding coastal position.
The fortress protected San Juan, a strategically important Spanish port on routes linking Europe and the Americas. Its six levels, artillery positions, steep setting, and walls as much as roughly 18 to 25 feet thick made it a formidable defense against seaborne attacks. The present form was largely completed in 1787.
UNESCO inscribed El Morro as part of the wider San Juan National Historic Site in 1983, alongside the city’s walls and other fortifications. Castillo San Marcos, Fort McHenry, and Fort Sumter are important historic American forts, but none is the San Juan fortress named in this question.