2 Best Sights in Old San Juan, San Juan

This huge stone fortress, built between 1634 and 1783, 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. National Park Service; it's a UNESCO World Heritage Site as well. Rangers conduct tours in Spanish and English.

At the northwestern tip of Old San Juan, El Morro (the promontory) was built by the Spaniards between 1539 and 1790. Rising 140 feet above the sea, the massive six-level fortress was built to protect the port and has a commanding view of the harbor. It is a labyrinth of cannon batteries, ramps, barracks, turrets, towers, and tunnels, through which you're free to wander. The cannon emplacement walls and the dank secret passageways are a wonder of engineering. A small but enlightening museum displays ancient Spanish guns and other armaments, military uniforms, and blueprints for Spanish forts in the Americas, although Castillo San Cristóbal has more extensive and impressive exhibits. There's also a gift shop. The fort is a National Historic Site administered by the U.S. National Park Service, and a UNESCO World Heritage Site as well. Various tours and a video are available in English.