Pisa - the Duomo
This is the cathedral of medieval Pisa, in Italy. It was built in the Romanesque style around 1064 AD. The rounded arches are typical of the Romanesque style. So is the floor plan of the building, with many little bits sticking out all over.
In some ways, the Pisa cathedral seems to be imitating the Islamic mosques that were being built at that time in Syria and Egypt and Spain. For instance, the front of the Pisa cathedral has lots of rows of columns, like the Great Mosque of Kairouan across the Mediterranean, which was built about 800 AD. And the Pisa cathedral has striped arches, like the Great Mosque of Cordoba, which was built about 700 AD.
Partly this was because the people of Pisa didn't have any money to spare when they were building this early cathedral. But it was also partly to show that the old polytheistic religion of the Romans was gone, powerless, and now Christianity was stronger. Christians could tear down old Roman buildings and use them for churches and the old Roman gods were powerless against them.
Video of the cathedral, the tower, and the baptistry
Both the mosque at Kairouan and the mosque at Cordoba also reuse Roman stones in a similar way, and for the same reasons.