Certainly one of the most interesting and cosmopolitan cities in the world is Istanbul. This is a city of many names and many cultures, both east and west, indeed its residents like to boast that it is the only major city where East meets West, because it sits on two continents. Be that as it may, Istanbul=Byzantium=Constantinople is a city of major importance to the history of Christianity, even though it existed only as a small Roman town in the first century A.D.
The signs are everywhere of the ancient Roman (see aqueducts above) and Christian presence. And if ancient history is not your thing, the food in this town is also stellar.
The art and architecture and artifacts of Christianity dominate various parts of this town, whether one is in Hagia Sophia (see below)
It has been said that those who refuse to learn from the past are doomed to repeat it. Whether that is always true, or not, there is certainly much to learn about Christianity in Istanbul, and along the way one learns about all sorts of other interesting subjects as well.