Why the Mezquita of Cordoba is special
The site of La Mezquita originally bore a church. Then, after the Moors conquered Cordoba in the 8th century, they demolished the church and built the Mezquita (mosque).
Over the centuries the Moors enlarged and enhanced La Mezquita into a vast, magnificent house of worship. They creatively used a variety of architectural styles including Persian, Mid Eastern Islamic, Roman and Gothic that together helped define Moorish architecture.
One of the most striking design elements of La Mezquita is the double-tiered "candy stripe" arches supported by sculptured pillars (see photo).

Both mosque and church
The Christians rewon Cordoba in the 13th century and promptly consecrated the edifice as a church but left much of the superb Moor-erected architecture intact. Their major alteration was building a cathedral within the mammoth mosque.
Today, the building is peacefully divided between Muslims and Catholics (who use the bizarrely situated cathedral).

What leading travel books say about the Mezquita of Cordoba
As you step inside the Mezquita, you'll face a forest of gleaming pillars of precious marble, jasper and onyx.
Europe
Fodor's

The Mezquita is so large you can easily get lost in it.
Europe
Frommer's