San Juan National Historic Site

San Juan National Historic Site (Spanish: Sitio Histórico Nacional de San Juan) in the Old San Juan section of San Juan, Puerto Rico, is a National Park Service-managed historic site which preserves and interprets the Spanish colonial-era fortification system of the city of San Juan, and features structures such as the San Felipe del Morro and San Cristóbal fortresses. This fortification system is the oldest European construction under United States jurisdiction and one of the oldest in the New World.

Source: Wikipedia — San Juan National Historic Site (CC BY-SA 4.0)