The Castillo de San Cristobal is a Spanish fort in San Juan, Puerto Rico. It was built by Spain to protect against land based attacks on the city of San Juan. It is part of San Juan National Historic Site.
Castillo de San Cristóbal is the largest fortification built by the Spanish in the New World. When it was finished in 1783, it covered about 27 acres of land and basically wrapped around the city of San Juan. Entry to the city was sealed by San Cristobal's double gates. After close to one hundred years of relative peace in the area, part of the fortification (about a third) was demolished in 1897 to help ease the flow of traffic in and out of the walled city.
This fortress was built on a hill originally known as the Cerro de la Horca or the Cerro del Quemadero, which was changed to Cerro de San Cristobal in celebration of the Spanish victories ejecting English and Dutch interlopers from the island of this name in the Lesser Antilles, then forming part of the insular territorial glacis of Puerto Rico.
Most of San Juan's fortified walls have guerites (sentry boxes, "garitas" to the locals) at various points. One of the guerites at Fort San Cristóbal is called "The Devil's Guerite" ("La Garita del Diablo"). This particular guerite is one of the oldest parts of the fort being built in 1634.