While El Morro protected San Juan from attacks by sea, Fort San Cristóbal was built to protect the city from land attacks. Its construction started in 1634, after the Dutch successfully—though briefly—took the city by land. By the late 1700s, San Cristóbal had become massive, with walls that encompassed the old city. As part of the San Juan National Historic Site, Fort San Cristóbal is a UNESCO World Heritage Site and a U.S. National Historic Site. Visitors can explore the tunnels and dungeon, walk the walls with their garitas (sentry boxes) and see the Santa Barbara chapel.