Why does a Renaissance cathedral sit inside the Great Mosque of Córdoba?

Because the building has had two lives. The Great Mosque of Córdoba was built in 785 on the orders of Abd al-Rahman I and expanded several times under his successors up to the late 10th century. In 1236, King Ferdinand III of Castile conquered Córdoba during the Reconquista, and the mosque became a Catholic cathedral dedicated to the Virgin Mary.

For nearly three centuries the structure itself underwent only minor modifications. Then in the 16th century a Renaissance nave and transept were inserted into its centre. The project, begun in 1523 and finished in 1607, was initiated by Bishop Alonso de Manrique and vigorously opposed by the city council, but the cathedral chapter won its case by petitioning Charles V. When Charles V later saw the result he is reputed to have been displeased, saying they had destroyed something unique in the world to build what could have been built anywhere.

What had they cut into? The mosque's most famous innovation, repeated in every Muslim-era expansion, was its rows of two-tiered arches in the prayer hall: a lower tier of horseshoe arches and an upper tier of semicircular ones, with stones alternating between red brick and white stone. By the end, in 987-988, Al-Mansur had extended the mosque sideways to the east, making it the largest mosque in the Muslim world outside Abbasid Iraq.

Today the building continues to serve as the city's cathedral, with Mass celebrated daily.