The San Juan National Historic Site is located in San Juan, Puerto Rico. It encompasses several historic structures, including the famous Castillo San Felipe del Morro and Castillo San Cristóbal. These fortifications were built by the Spanish in the 16th and 17th centuries to protect the city from invasions. Visitors to the site can explore the impressive walls, tunnels, and scenic views of the Atlantic Ocean. The area is a UNESCO World Heritage Site, highlighting its cultural significance and historical importance in the context of colonial history and military architecture.