What is the least known fact about the Roman Empire?

The Roman Empire was the only Empire to have 2 capital cities. Rome was the original capital, but when the Roman Empire reached it's peak and expanded, it had to build another capital to serve as the center. That capital was Byzantium. It was renamed Constantinople in honor of the Roman Emperor. The capital was moved from Rome to Byzantium (now Istanbul) because Rome was in danger. After Rome was moved, Rome continued to be the capital of the western empire while Byzantium was the capital of the eastern empire.