Castillo San Felipe del Morro, most commonly known as El Morro (The Promontory), is a large fortress and citadel in the historic district of Old San Juan, Puerto Rico. Commissioned by Charles I of Spain in 1539, it was first built as a fortified tower in honor of Philip II, who oversaw its expansion into a hornwork fort by 1595. Over the next 200 years, especially in the reign of Charles III, El Morro continued to be developed to reach its current form in 1787. Rising 140 ft from the Atlantic shoreline with 18 to 25 ft thick walls, it stands on a steep, rocky headland promontory on San Juan Islet guarding the entry to San Juan Bay, the harbor of Old San Juan. El Morro, alongside La Fortaleza, San Cristóbal, El Cañuelo, and other forts part of the Walls of Old San Juan, protected strategically and militarily important Puerto Rico, or La Llave de las Indias , from invasion by competing world powers during the Age of Sail. It was designated a World Heritage Site by UNESCO in 1983.

Old San Juan is a historic district located at the "northwest triangle" of the islet of San Juan in San Juan. Its area roughly correlates to the Ballajá, Catedral, Marina, Mercado, San Cristóbal, and San Francisco sub-barrios (sub-districts) of barrio San Juan Antiguo in the municipality of San Juan, Puerto Rico. Old San Juan is the oldest settlement within Puerto Rico and the historic colonial district of the city of San Juan. This historic district is a National Historic Landmark District named Zona Histórica de San Juan and is listed on the United States National Register of Historic Places as the Old San Juan Historic District. Several historical buildings and structures, particularly La Fortaleza, the city walls, and El Morro and San Cristóbal castles, have been inscribed in the UNESCO World Heritage Site list since 1983. Historically the mixed-use commercial and residential real estate in the main streets of Calle Cristo and Calle Fortaleza from Calle Tanca to the Governor’s Mansion is the most valuable in the area and it has kept its value and increased steadily through several years despite the past economic turmoils.