Castillo San Cristóbal (San Juan) in the context of "Medieval fortification"

San Juan (/ˌsæn ˈhwɑːn/ san WHAHN, Spanish: [saŋ ˈxwan]; Spanish for "Saint John") is the capital city and most populous municipality in the Commonwealth of Puerto Rico, an unincorporated territory of the United States. As of the 2020 census, it is the 57th-most populous city under the jurisdiction of the United States, with a population of 342,259. San Juan was founded by Spanish colonists in 1521, who called it Ciudad de Puerto Rico (Spanish for "Rich Port City").

Puerto Rico's capital is the second oldest European-established capital city in the Americas, after Santo Domingo, in the Dominican Republic, founded in 1496, and is the oldest European-established city under United States sovereignty. Several historical buildings are located in the historic district of Old San Juan; among the most notable are the city's former defensive walls, Fort San Felipe del Morro and Fort San Cristóbal, and La Fortaleza, the oldest executive mansion in continuous use in the Americas. These historic sites were declared a UNESCO World Heritage Site in 1983.

La Santa María de la Inmaculada Concepción (Spanish: [la ˈsãn̪.t̪a maˈɾi.a ð̞e̞ la ĩm.ma.kuˈla.ð̞a kõn̟.θeβ̞ˈθjõn] lit. 'The Holy Mary of the Immaculate Conception'), or La Santa María (Spanish: [la ˈsãn̪.t̪a maˈɾi.a]), originally La Gallega (Spanish: [la ɡaˈʝe.ɣ̞a]), was the largest of the three small ships used by Christopher Columbus in his first expedition across the Atlantic Ocean in 1492, with the backing of the Spanish monarchs. Her master and owner was Juan de la Cosa.

In 1492 the ship ran aground on a sand bar near modern day Cap-Haïtien of the island of Hispaniola. The wood of the ship was stripped and used to make a wooden fort at Limonade, and one anchor survives to the present day in a Museum in Haiti. In the 19th and 20th century several replicas have been made of varying nature, as the size and details of the original are unknown.

Old San Juan (Spanish: Viejo San Juan) is a historic district or quarter 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 Puerto Rico Registered Historic Zone and a National Historic Landmark District, Old San Juan Historic District, and is also listed on the United States National Register of Historic Places. 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.

The Walls of Old San Juan (Spanish: Murallas del Viejo San Juan) is a defensive city wall that surrounds the western end of the San Juan Islet, site of the historic district of San Juan, Puerto Rico. This defensive wall system was built between the 16th and 18th centuries to protect the city and the Bay of San Juan, a highly strategic point in the Caribbean. These walls, along with the defensive fortresses in Old San Juan (La Fortaleza, El Morro and San Cristóbal), form part of the UNESCO World Heritage Site-designated San Juan National Historic Site as they are characteristic examples of the historic methods of construction used in military architecture which adapted European designs and techniques to the special conditions of the Caribbean port cities.

San Juan remains the only walled city under the United States jurisdiction, with walled cities like Boston, Charleston and New Orleans no longer having walls after the 18th century, although St. Augustine, Florida, still retains several wall remnants. Examples of other North American walled cities that retain their ramparts are Quebec City in Canada and Campeche in Mexico. The bartizans (garitas) found along the walls are considered emblematic of the city of San Juan and a symbol of historic heritage preservation in Puerto Rico.

Castillo San Felipe del Morro (English: Promontory Castle of Saint Philip), most commonly known as El Morro (The Promontory), is a large fortress and citadel in the Old San Juan historic quarter of San Juan, the capital municipality of Puerto Rico. Commissioned by King Charles I of Spain in 1539, it was first built as a fortified tower in honor of King Philip II, who oversaw its expansion into a hornwork bastion fort by 1595. Over the next 200 years, especially in the reign of King Charles III, El Morro continued to be developed to reach its current form in 1787. Rising 43 metres (140 ft) from the Atlantic shoreline with 5.5 to 7.6 metres (18 to 25 ft) thick walls, the six-leveled edifice 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 (The Key to the Indies), from invasion by competing world powers during the Age of Discovery and Sail. It was designated a World Heritage Site by UNESCO in 1983.

La Fortaleza (English: "the fortress"), officially the Palacio de Santa Catalina ("Saint Catherine's Palace"), is the official residence and workplace of the governor of Puerto Rico. Located in the historic quarter of Old San Juan in the capital municipality of San Juan, it has served as the governor’s residence since the 16th century, making it the oldest executive mansion in continuous use in the New World. Built as a medieval fortress from 1533 to 1540 by orders of King Charles I of Spain, and remodeled to its present Neoclassical style in 1846 by orders of Governor Rafael Arístegui y Vélez, it was the first fortification erected by the Spanish on San Juan Islet to defend San Juan Bay, the harbor of Old San Juan. Alongside El Morro, San Cristóbal, and other forts part of the Walls of Old San Juan, it protected strategically and militarily important Puerto Rico, or La Llave de las Indias (The Key to the Indies), from invasion by competing world powers and harassment by privateers and pirates during the Age of Discovery and Sail. It was designated a World Heritage Site in 1983.

Situated in the western end of San Juan Islet in the Old San Juan historic quarter, La Fortaleza, seat of the executive branch, is about 1 mile (1.6 km) from the Capitol of Puerto Rico, seat of the legislative branch, in the center of the Islet in the Puerta de Tierra historic district, and 2 miles (3.2 km) from the Supreme Court Building, seat of the judicial branch, in the eastern end of the Islet in Puerta de Tierra.

San Cristóbal is one of the seven subbarrios of the San Juan Antiguo barrio in the municipality of San Juan in Puerto Rico. San Cristóbal, named after Castillo San Cristóbal, is one of the six subbarrios which form part of the Old San Juan Historic District. Half of La Perla is located within the borders of the San Cristóbal subbarrio, the other half is located in the neighboring subbarrio, Mercado.

San Juan National Historic Site (Spanish: Sitio Histórico Nacional de San Juan) in the Old San Juan section of San Juan, Puerto Rico, is a National Park Service-managed historic site which preserves and interprets the Spanish colonial-era fortification system of the city of San Juan, and features structures such as the San Felipe del Morro and San Cristóbal fortresses. This fortification system is the oldest European construction under United States jurisdiction and one of the oldest in the New World. This national historic site, together with La Fortaleza, have been designated a UNESCO World Heritage Site since 1983.