Built with coral mined from the region’s reefs, the imposing Castillo de San Felipe de Barajas is considered the greatest fortress constructed by the Spaniards in the New World. Built in several phases, beginning in 1656, atop the 40-meter-high San Lázaro hill, the Castillo de San Felipe ranks as one of the great military wonders of the world. It was Sir Francis Drake’s attack of 1586—the British pirate pillaged the town for some 50 days—that focused the attention of the Spanish Crown, which then embarked on an unprecedented mission to bolster the city’s defenses. After the fort was enlarged in 1762, Cartagena became impregnable. The most important port (and the most heavily fortified city in the New World), Cartagena was the nexus for the Spanish flotillas that transported gold and silver via Havana to Spain. The castle’s labyrinthine, rather ghoulish, tunnels are open for self-guided audio tours (available in English) which relate the castle’s seminal architectural accomplishments; famed engineer Antonio de Arévalo designed the tunnels such that any noise would carry for the entire length of the tunnels to alert the Spanish to enemy advances.