Trier, Germany’s oldest city, lies at the head of the scenic Mosel Valley near the Luxembourg border. An ancient Roman capital, Trier brags that it was inhabited for 1,300 years before the Romans arrived. Today Trier feels young and thriving. A short stop here offers you a look at Germany’s oldest Christian church, one of its most enjoyable market squares, and its best Roman ruins.Founded by Augustus in 15 B.C., Trier was the Roman “Augusta Treverorum” for 500 years. For most of the fourth century, this city of 80,000 — with a four-mile-long wall, four great gates, and 47 round towers — was the favored residence of Roman emperors. Emperor Constantine used the town as the capital of his fading western Roman Empire.