Not only is Trier a Roman city, it is also the oldest city in Germany. It began around a wooden bridge from 18 B.C. at the site of a ford used by the Celtic Treveri and soon occupied the wide valley with an extensive grid pattern of streets on the site of sporadic Celtic settlements. Because it had been founded under Caesar Augustus in the area of the Treveri, it was called Augusta Treverorum and later shortened to Treveri. Facing the threat of a Germanic invasion, the Romans finally built a city wall around A.D. 180, 6.4 kilometers long, which ultimately had five gates. The remains of the gates at the Amphitheater and the Roman Bridge can still be seen and the Porta Nigra is almost perfectly preserved.