La Fortaleza and San Juan National Historic Site in Puerto Rico is a UNESCO World Heritage Site located in Puerto Rico, an unincorporated territory of the United States in the Caribbean. The world heritage site consists of several historic defensive structures built by the Spanish Empire between the 16th and 20th centuries to defend the strategically located colonial city of San Juan and its bay from foreign attacks. These fortifications are among the oldest European-built defensive systems and largest of its kind in the Western Hemisphere. In addition to its historic importance the site is notable for its architectural significance as one of the most prominent military adaptations of Renaissance and Baroque architecture in the Americas.