Castillo San Cristóbal sprawls across 27 acres and was built to stop an attack by land. Its designers were gloomy planners: the tunnels are grooved to hold gunpowder, so defenders could blow them up and bury any invaders inside.

The fort’s record is oddly modest. In 1797 a British force under Sir Ralph Abercromby gave up before reaching its doors. In 1855 rebels held it for 24 hours and turned its cannons toward the city. In 1898 the Spanish surrendered within a day of a US naval attack. Soldiers’ drawings still mark the dungeon walls.

VoiceMap’s self-guided audio tours let the fort tell its own story in Talking Buildings of Old San Juan, and use its gate, once the city’s land entrance, to trace Spanish and American rule through Puerto Rico’s music in a reggaetón history walk.