Before that, it had been split in half, originally by Diocletian, I believe. You may remember him as one of the nastier persecuters of Christians. This division did become permanent, and there were emperors for the eastern half and the western half (and they had sort of assistants, not unlike Dwight Shroot). Although the west declined into the manor style situation we know of feudalism today, the East managed to hold on a bit better. This eventually came to be known as the Byzantine Empire. The capital was Constantinople (present day Istanbul, built up by Constantine, the Roman emperor who made Christianity legal and ultimately was very important for Christianity’s acceptance in the empire).
Constantinople was to be a Christian city, and so Constantine did much constructing to this end. I believe this was in the fourth century (300’s), off the top of my head. The Hagia Sophia was built, burnt down a few times, and was finally rebuilt by Justinian in 537. It is still absolutely amazing. It was considered the greatest church in Christendom until 1453 (when Mehmet the conqueror took over Constantinople and made it a muslim city).
I highly recommend seeing it at least once in your lifetime.