The historical record describes the Roman Empire subduing the Treveri in the and establishing Augusta Treverorum about 16 BC. The name distinguished it from the empire's many other cities honoring the first emperor Augustus. The city later became the capital of the province of Belgic Gaul; after the Diocletian Reforms, it became the capital of the prefecture of the Gauls, overseeing much of the Western Roman Empire. In the 4th century, Trier was one of the largest cities in the Roman Empire with a population around 75,000 and perhaps as much as 100,000. The Porta Nigra ("Black Gate") dates from this era. A residence of the Western Roman Emperor, Roman Trier was the birthplace of Saint Ambrose. Sometime between 395 and 418, probably in 407 the Roman administration moved the staff of the Praetorian Prefecture from Trier to Arles. The city continued to be inhabited but was not as prosperous as before. However, it remained the seat of a governor and had state factories for the production of ballistae and armor and woolen uniforms for the troops, clothing for the civil service, and high-quality garments for the Court. Northern Gaul was held by the Romans along a line from north of Cologne to the coast at Boulogne through what is today southern Belgium until 460. South of this line, Roman control was firm, as evidenced by the continuing operation of the imperial arms factory at Amiens.