Historians have speculated that Córdoba, a sun-drenched city in southern Spain, once boasted the largest population anywhere in the world. In fact, some of the greatest thinkers of ancient Rome and the Middle Ages—Seneca, Lucan, and Maimonides, to name a few—were cordobeses. As you might expect, there’s no shortage of historical sites to discover in Córdoba, but one in particular eclipses the rest: the Great Mosque. Dating to the 10th century, this sprawling compound was a symbol of Islamic grandeur as the city rose to become the European capital of culture and the arts. Since its initial construction, the mosque has undergone several controversial renovations, the most significant taking place in the 16th century, when dozens of original arches were demolished to make way for a Renaissance cathedral nave.