Mezquita, Cordoba
Just when we thought we'd seen the best cathedral/mosque Spain has to offer...the Mezquita tops the lot! First a Visigoth/Christian church, a mosque was begun on the site in 785 and shared with Christians who continued to worship there. It had several additions to the mosque as the Muslim population grew over the 600 years of their dominance in Cordoba. Then, amazingly, after the Christian reconquest in the late 12thC, naves and chapels were built in the Catholic style within the mosque, instead of pulling it down as was done elsewhere. Then between 1600 and 1750 a complete cathedral was constructed within the mosque, preserving most of its beauty.