Trier has 3 sets of excavated Roman baths. Those at Viehmarkt ('cattle market') are the oldest (dating from 80AD) and the smallest, most popular in the 3rd and 4th centuries. Asfter Roman rule ceased they gradually fell into disrepair and, as was usual with 'Roman ruins' in all European countries, stones from the bath structures were scavenged and re-used for building elsewhere in the town.
The remains were unexpectedly discovered in 1987 when a car park was being built under the Viehmarkt. A glass cube, designed by architect Oswald Ungers, protects the excavated baths.
The baths are open for visitors year-round but closed on Mondays. Opening times are 9am to 5pm. Admission 3 euro.
I didn't bother to go inside. The glass cube allows you to see the remains quite well (especially if you squish your nose up to the glass!) an I was more interested in visiting the vast Kaiserthermen baths.