Puerto Rico is an Estado Libre Asociado, that is, a commonwealth territory of the United States and its spirited capital, San Juan, was founded in 1521. Though it is now the oldest capital city flying the US flag, it was once the strategic center of the Spanish Empire in the Caribbean, and maintains the largest and best-preserved complex of Spanish colonial architecture in the region. Cathedrals, museums, and stately Spanish squares abound, creating an ambience of romance and colonial grandeur.