This precedent had already been established by Gallienus in 260, who moved the imperial court to Mediolanum in response to a suspected future attack by the usurper Postumus as well as defending Italy from the ravages of the Alamanni. Rome was increasingly seen to be too distant a residence for the emperor when troubles could erupt along any of the borders of the empire. In the west, Mediolanum was seen to be a much better strategic city for the emperor to be based at, as it gave good access through the Alps northwards to both the Danubian provinces in the east as well as the Rhine provinces and Gaul to the west. Further, it was well positioned to guard against incursions through the alpine passes. This decision was confirmed when Diocletian established the Tetrarchy, and his colleague Maximian informally established Mediolanum as the senior western emperor's official residence. Diocletian, conscious that the Persian threat to the eastern provinces required a continuous imperial presence, placed his eastern capital in the city of Nicomedia. Meanwhile, the Caesars also had imperial residences – Constantius Chlorus was based at Augusta Treverorum, while Galerius sited his residence at Sirmium. After the collapse of the Tetrachy, Constantine I at first placed his imperial capital at Ulpia Serdica before erecting a new imperial capital on the site of the old Greek city of Byzantium. Naming it Constantinople, it finally replaced Nicomedia as the site of the emperor's residence in the east in 330. Constantinople would remain the capital of the eastern provinces throughout the period of the Dominate. In the west, Mediolanum continued to be the imperial residence until the repeated invasions by Alaric I forced the western emperor Honorius to relocate to the strongly fortified city of Ravenna in 402. Ravenna remained the western imperial capital until the loss of Italy in 476. Although Rome was reincorporated into the empire by Justinian I in 540, it was Ravenna which was selected as the official residence of the Exarch, the governor who represented the emperor in Italy. Ravenna would retain this position until 751, when the Byzantine Empire finally lost the central Italian provinces to the Lombards. Intensification of coloni use and the origins of serfdom Large Roman landowners increasingly relied on Roman freemen, acting as tenant farmers to provide labor. The status of these tenant farmers, eventually known as coloni, steadily eroded. Because the tax system