During the Spanish colonial times most of the urban population resided in what is now known as Old San Juan. This sector is located on the western half of a small island called the Isleta de San Juan, which is connected to the mainland by bridges and a causeway. The small island, which comprises an area of 47 mi² (122 km²), also hosts the working class neighborhood of Puerta de Tierra and most of Puerto Rico's central government buildings, including those of the Commonwealth's Capitol.
The main central part of the city is characterized by narrow cobblestone streets and picturesque colonial buildings, some of which date back to the 16th and 17th century. Sections of the old city are surrounded by massive walls and several defensive structures and notable forts. These include the 16th century Fort San Felipe del Morro and 17th century Fort San Cristóbal, both part of San Juan National Historic Site, and the 16th century El Palacio de Santa Catalina, also known as La Fortaleza, which serves as the governor's mansion. Other buildings of interest predating the 20th century are the Ayuntamiento or Alcaldía (City Hall), the San José Church (1523) and the adjacent Hotel El Convento, the former house of the Ponce de León family known as Casa Blanca, the Teatro Tapia, the former Spanish barracks (now Museum of Ballajá), La Princesa (former municipal jail, now a history museum), and the municipal cemetery of Saint María Madgalena of Pazzis, located just outside the city walls. The Cathedral of San Juan Bautista (construction began in the 1520s) is also located in Old San Juan, and contains the tomb of the Spanish explorer and settlement founder Juan Ponce de León.