Aachen's thermal springs were the reason the Romans built here, the reason Charlemagne loved the city, and the reason the name exists: "Aix" and "Aachen" both derive from the Latin aquae, waters. The Carolus Thermen is the modern thermal complex built over these springs, with water rising at 74°C and cooled to bathing temperature before entering several indoor and outdoor pools. The mineral content is exceptionally high — sulphurous, chalybeate, and slightly saline — giving the water a noticeable warmth on the skin beyond temperature.
The complex near the old town is comfortable rather than glamorous, but the setting is genuine: you are bathing in the same waters that Charlemagne used, in the shadow of the Dom. A good option on a cold afternoon or as part of a longer Aachen visit.