From the 8th century BC the Celtic peoples inhabited most of present-day Germany but by about 500 BC Germanic tribes had pushed their way north and settled in the Celtic lands. The expanding Roman Empire established its boundaries along the Rhine and the Danube rivers but attempts to move further east had to be abandoned after the Roman provincial Governor Varius was defeated in AD 9 by the Germanic forces under Arminius. For the next thousand years the towns of Trier, Regensburg, Augsburg, Mainz and Cologne, founded by the Romans, formed the main centres of urban settlement. Christianity was introduced under Emperor Constantine and the first bishopric north of the Alps was established in Trier in AD 314.