Castillo San Cristóbal is an enormous and fascinating historical site in Old San Juan, part of the San Juan National Historic Site and a UNESCO World Heritage Site. It was the largest fortification built by the Spanish in the Americas, designed primarily to protect the city from land attacks. You will be able to see The Fortification's Design: Explore the ingenious, multi-layered design known as "defense-in-depth," which features several interconnected forts, bastions, and outer defenses. Panoramic Views: Enjoy breathtaking, sweeping views of the Atlantic Ocean, the coastline, the modern city of San Juan, and the historic rooftops of Old San Juan. It's a goldmine for photographers. The Tunnels and Underground Passages, Wander through a maze of tunnels and underground passages, which were strategically built for troop movement, counter-mining (placing explosives beneath an attacking enemy), and protection.