Trier was the capital of the tribe of the Treveri, one of the most powerful political units in Gaul. After the Roman conquest, it became an important city, seat of the procurator of Gallia Belgica and -much later- capital of the Praefecture of Gaul.
I have visited the city several times, but usually without my camera. Still, my friends Marco and Richard have made several photos, and in the end, I could create several webpages about this nice town. There is a brief intro, and there are pages on the Imperial Baths, the famous Porta Nigra, the Basilica, and the bridge which is about 1800-1900 years old and still in use. Plus a couple of minor subjects, like the Amphitheater.
There are two excellent museums in Trier, which you simply must visit: the Landesmuseum shows many stone monuments from the city and its surroundings, and has some lovely mosaics and frescos; the Bischöfliches Dom- und Diözesanmuseum has a lot of early Christian art, including the most splendid frescos I have ever seen (with the possible exception of the Sistine Chapel). I hope to post photos of those mosaics later.
Another article on Roman Trier can be found here.