Castillo San Cristóbal

The Dutch came by sea in 1625 and took San Juan from the land side. Spain's answer, built over the next 150 years, was Castillo San Cristóbal — the largest fortification Spain ever raised in the Americas, covering 27 acres on the eastern edge of Old San Juan. Three levels of bastions, a dry moat, and a network of tunnels designed to move troops and detonate gunpowder beneath an advancing enemy. In 1797, it worked: a British invasion force was halted before it reached the walls. One sentry box here, the Garita del Diablo, carries its own story — soldiers, local tradition holds, disappeared from inside it without explanation.