San Juan National Historic Site

The Spanish began building here in the 16th century to protect their treasure fleets, and what they raised endures as the oldest European construction under United States jurisdiction. The fortification system — Castillo San Felipe del Morro, Castillo San Cristóbal, and three miles of city wall — held the entrance to San Juan Bay through centuries of imperial rivalry and earned UNESCO World Heritage designation in 1983. San Cristóbal, the largest fortification ever built in the New World, still contains its dungeons, barracks, and tunnels. The National Park Service runs it all, and the walls are worth the walk.