Even the Roman cities began to change in character. The large cities of classical antiquity slowly gave way to the smaller, walled cities that became common in the Middle Ages. These changes were not restricted to the third century, but took place slowly over a long period, and were punctuated with many temporary reversals. In spite of extensive reforms by later emperors, however, the Roman trade network was never able to fully recover to what it had been during the Pax Romana (27 BCE – CE 180). This economic decline was far more noticeable and important in the western part of the empire, which was also invaded by barbarian tribes several times during the century. Hence, the balance of power clearly shifted eastward during this period, as evidenced by the choice of Diocletian to rule from Nicomedia in Asia Minor, putting his second in command, Maximian, in Milan. This would have a considerable impact on the later development of the empire with a richer, more stable eastern empire surviving the end of Roman rule in the west.