For me, Trier was a great visit because of it's Roman history.
I camped at this stellplatz:
Reisemobilpark Treviris, In den Moselauen, 54294 Trier, Germany
(If you have a problem with the pay machine, ring the number beside it, as the manager also runs the campsite a couple of hundred metres north, and he'll be there in a flash.)
I cycled north along the Moselle, crossing into the town via the Romerbrucke. When I visited I also descended beneath the Amphitheatre and a guide was dressed in a Roman tunic, pretending to be a soldier gladiator. His audience (a party of young schoolchildren) will never forget him.