In Baden-Baden, you can visit the preserved remains of Roman baths from ancient times, which were discovered during the construction of the Friedrichsbad in the 19th century. In addition to the comparatively simple soldiers' baths, an inscription attests that Emperor Caracalla (188–217) expanded the far more luxurious imperial baths. In any case, the Roman thermal system was highly advanced: A hot-air heating system, for instance, ensured that the walls and floor were consistently heated and that a succession of bathing rooms with different temperatures was possible.
Roman finds before the entrance to the bath ruins underneath the Friedrichsbad, color photograph, 2016, photographer: Moritz Grenke; source: http://www.stadtwiki-baden-baden.de/wiki/Datei:R%C3%B6mischeFundst%C3%BCcke.jpg/, Creative Commons Attribution-ShareAlike 3.0 Germany license (CC BY-SA 3.0 DE).