Castillo de San Cristóbal is the largest fortification built by the Spanish in the New World. When it was finished in 1783, it covered about 27 acres of land and basically wrapped around the city of San Juan. Entry to the city was sealed by San Cristóbal's double gates. After close to one hundred years of relative peace in the area, part of the fortification (about a third) was demolished in 1897 to help ease the flow of traffic in and out of the walled city.

Old San Juan is the oldest settlement within Puerto Rico and is the historic colonial section of San Juan, Puerto Rico. It was listed on the U.S. National Register of Historic Places in 1972 as "Zona Histórica de San Juan" (San Juan Historic Site),[2] and declared a National Historic Landmark District in March 2013.

This restored 19th-century esplanade is the place to begin your stroll through Old San Juan, because of its central location and its inexhaustible charm.

Located on an isolated point with white lime cliffs surrounded by lagoons and marshes, Cabo Rojo, built in 1882, marks the southeast entrance from the Caribbean Sea through Mona Passage into the Atlantic Ocean. Cabo Rojo's lighthouse has simple decorative elements including an unelaborated cornice that is repeated in the tower.

Puerto Rico’s geographic position at the western edge of the Caribbean made San Juan one of the key frontier outposts of Spain’s West Indies dominions. San Juan National Historic Site includes Castillo San Cristóbal, Castillo San Felipe del Morro, Fortín San Juan de la Cruz known locally as El Cañuelo, and three fourths of the city walls.