Thanks to its deep history, the Trier of today holds a wealth of ancient sites. It's also an important university town, and accordingly boasts a surprisingly rich modern cultural landscape for a city of its size (just over 100,000 residents).
Its roots reach back to at least 400 BC, by which time a Celtic tribe, the Treveri, had settled the Trier Valley. Eventually Julius Caesar's legions arrived at this strategic point on the river, and Augusta Treverorum ("the town of [Emperor] Augustus in the land of the Treveri") was founded in 16 BC. It was described as an opulent city, as beautiful as any outside Rome.
Around AD 275 an Alemannic tribe stormed Augusta Treverorum and reduced it to rubble. But it was rebuilt in even grander style and renamed Treveris. Eventually it evolved into one of the leading cities of the empire, and was promoted to "Roma secunda" (a second Rome) north of the Alps. As a powerful administrative capital it was adorned with all the noble civic buildings of a major Roman settlement, as well as public baths, palaces, barracks, an amphitheater, and temples. The Roman emperors Diocletian (who made it one of the four joint capitals of the empire) and Constantine both lived in Trier for years at a time.
Trier survived the collapse of Ancient Rome and became an important center of Christianity and, ultimately, one of the most powerful archbishoprics in the Holy Roman Empire. The city thrived throughout the Renaissance and baroque periods, taking full advantage of its location at the meeting point of major east–west and north–south trade routes and growing fat on the commerce that passed through.