Construction on this huge fortress began in 1634. The structure was reengineered in the 1770s, and is one of the largest ever built in the Americas by Spain. Its walls rise more than 150 feet (46m) above the sea—a marvel of military engineering. San Cristóbal protected San Juan against attackers coming by land in conjunction with El Morro, which protected the city from invaders by seas. The two are linked by a half-mile (.8km) of monumental walls and bastions filled with cannon-firing positions. A complex system of tunnels and dry moats connects the center of San Cristóbal to its “outworks,” defensive elements arranged layer after layer over a 27-acre (11-hectare) site. To get the full scope, be sure to look at the scale model on display. Like El Morro, the fort is administered and maintained by the U.S. National Park Service. Be sure to see the Garitadel Diablo (the Devil’s Sentry Box), one of the oldest parts of San Cristóbal’s defenses, and famous in Puerto Rican legend. The devil himself, it is said, would snatch away sentinels at this lonely post at the edge of the sea. In 1898, cannons on top of San Cristóbal fired the first shots of the Spanish-American War in Puerto Rico during an artillery duel with a U.S. Navy fleet. Park rangers lead hour-long tours for free here, but wandering on your own is fun.