Old San Juan is a historic district located at the "northwest triangle" of the islet of San Juan. Its area roughly correlates to the Ballajá, Catedral, Marina, Mercado, San Cristóbal, and San Francisco sub-barrios 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 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.