The Chora Church is a Byzantine church (and later an Ottoman mosque, but now a museum) going back as far as the 413 AD. The majority of the building dates from 1077, but its beautiful frescos and mosaics were added in the early 1300's. Many of the images are not from western Christian gospels, but reference apocryphal texts. The place is stunning, but dark and difficult to get good photos in. There's a nice collection of interior images here, and the church museum has its own website.