This is the mosque of Cristo de la Luz in Toledo, Spain. The square part of the building on the right hand side is the original mosque (mesquita), with the three horseshoe arches on the main level, and the row of striped keyhole windows above. It’s small, only 8m square, and was founded in 999 by Ahmad ibn Hadidi, and known as the Mezquita Bab-al-Mardum. When Toledo was taken by Christian forces in 1085, the mosque was converted to Christian church, renamed Cristo de la Luz, with an added rounded apse. You can see the windows of the apse on the left are a different shape, with pointed window arches.
The brick work of the outside is almost seamless between the Islamic/Moorish section of 999 and the Christian of a century later. Certainly the craftspeople of one century knew the work of the previous–they may have been descendants of the same workers. The Moors were forced to convert to Christianity, or leave. Those who stayed became the Mudejar, the converted. They created a new fusion of architecture and art, joining Muslim and Christian forms.
Inside the mosque, the columns holding up the 9 small domes of the roof are Visigoth. Spolia. From the Latin for spoils of war. Older stone works that are incorporated into new structures, either from a need for building materials, or a desire to visibly overtake the old structure in a literal way, proving the new society’s dominance. Or, as I imagine it, a carver saying, that’s a nice bit of stone work, that is. Best keep it.