Nothing can prepare you for the forest of columns and arches inside Córdoba's mesmerizing Mezquita. One of the world's greatest works of Islamic architecture, founded in 786 by Emir Abd ar-Rahman I. When Córdoba fell to the Christians in 1236, they planted a cathedral right in the middle — a surreal juxtaposition that's now a UNESCO site. 856 columns of jasper, onyx, marble and granite support double-tiered arches in stripes of red and white.