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.

Remains of a shell shed and a fishing site were uncovered in 1937 (near the modern-day Puerto Rico National Guard Museum) in by archaeologist Irving Rouse from the University of Yale.

[4] The town of Puerto Rico de San Juan Bautista was officially founded on July 20, 1521.

[6][7] With the establishment of a port and other militaristic infrastructure, the original colonial settlement of San Juan kept growing in size until the 18th century when the town of El Roble (today Río Piedras) took its place as the largest urban center in the region.

[11] Isleta de San Juan is connected to the main island of Puerto Rico by three bridges and a causeway.