Castillo San Felipe del Morro in the context of "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.

A fortification (also called a fort, fortress, fastness, or stronghold) is a military construction designed for the defense of territories in warfare, and is used to establish rule in a region during peacetime. The term is derived from Latin fortis ("strong") and facere ("to make").

From very early history to modern times, defensive walls have often been necessary for cities to survive in an ever-changing world of invasion and conquest. Some settlements in the Indus Valley Civilization were the first small cities to be fortified. In ancient Greece, large cyclopean stone walls fitted without mortar had been built in Mycenaean Greece, such as the ancient site of Mycenae. A Greek phrourion was a fortified collection of buildings used as a military garrison, and is the equivalent of the Roman castellum or fortress. These constructions mainly served the purpose of a watch tower, to guard certain roads, passes, and borders. Though smaller than a real fortress, they acted as a border guard rather than a real strongpoint to watch and maintain the border.

San Juan Islet (Spanish: Isleta de San Juan) is a 3-square-mile (7.8 km) islet or small island on San Juan Bay in the Atlantic coast of northern Puerto Rico. Home to Old San Juan, it is the site of the oldest permanent European settlement in Puerto Rico (1521), and the second-oldest European settlement in the West Indies after Santo Domingo (1496). Due to its strategic location in the Caribbean during the Spanish colonization of the Americas, it is home to a city wall and a number of militaristic buildings such as El Morro Castle. Today, it is also home to many of Puerto Rico's government buildings such as the territory's capitol building (El Capitolio).

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 Cristóbal (English: Saint Christopher Castle) is a fortress in the Old San Juan historic quarter of San Juan, the capital municipality of Puerto Rico, known for being the largest fortification built by the Spanish in the New World. Dating back to defense expansions following attacks by the English in 1598 and Dutch in 1625, it was first constructed in 1634 as the Caballero de San Miguel (Saint Michael Cavalier) and the Garita del Diablo ("devil’s sentry box"). Over the next 150 years, especially in the reign of King Charles III of Spain, it continued to be developed into a bastion fort with outer ravelins and batteries to reach its largest size in 1783. Rising 150 ft from the Atlantic shoreline, the three-level edifice stands on a hill at the northern coastline of San Juan Islet, guarding the land entry to Old San Juan. Alongside El Morro, La Fortaleza, 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 during the Age of 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.