Constantinople

As you may have learned from They Might Be Giants (or the oldsters may have learned from the Four Lads), Istanbul was formerly known as Constantinople, capital of the Byzantine empire. As such, they have some great old Roman stuff, and even some churches. The Chora Church, pictured above, was a highlight, with ancient frescoes and mosaics on every wall (well, surviving on many walls, surely once literally on every wall). The Hagia Sofia was once a church, and also had some amazing mosaics of saints and virgins and such, which for reasons that remain unknown were not destroyed when it was converted to a mosque. And lastly, there is a whole mosaic museum dedicated to large mosaic floors left behind by some emperor, including pictures of monkeys wearing backpacks and bothering birds with sticks. Detail below from the museum, but not the monkey-with-a-backpack-and-a-stick part.