Capitals of the Roman Empire: Constantinople & Rome
Constantinople at first had much in common with the temporary capitals of the 2nd and 3rd century CE and the tetrarchic capitals. It was an existing city of medium size, well located on the road network, and unlike most of them, it was also well connected by water. It sat on the Sea of Marmara, which opened onto the Black Sea to the north and the Mediterranean to the south. It was close enough to the Danube frontier to keep track of war on that front, and not too far from the territories most often disputed with Persia.