This huge stone fortress, built between 1634 and 1790, guarded the city from land attacks from the east. The largest Spanish fortification in the New World, San Cristóbal was known in the 17th and 18th centuries as the Gibraltar of the West Indies. Five freestanding structures divided by dry moats are connected by tunnels. You're free to explore the gun turrets (with cannon in situ), officers' quarters, re-created 18th-century barracks, and gloomy passageways. Along with El Morro, San Cristóbal is a National Historic Site administered by the U.S. Park Service; it's a World Heritage Site as well. Rangers conduct tours in Spanish and English.