San Juan National Historic Site

The oldest European construction under United States jurisdiction wasn't built to impress — it was built to survive. San Juan National Historic Site preserves the Spanish colonial fortification system that defended San Juan Bay from the 16th century onward: El Morro at the harbor's edge, Castillo San Cristóbal — the largest fortification ever built in the New World — and three miles of city wall. Together with La Fortaleza, they've held UNESCO World Heritage designation since 1983. Come for the engineering. The stakes that produced it are still legible in the stone.