2 days back when you were asking about "castles" and "Bavaria," and mardee surprisingly suggested Gengenbach, you responded with genuine interest in Gengenbach, partly because it would position you well for your journey to Paris later. I encouraged you to look elsewhere for better castle options.
This thread makes no mention of castles or your trip to Paris. Assuming that castles aren't critical at this point, I'll just remind you that you were onto something with Gengenbach, a great small, historic town - south of Stuttgart, and on the Kinzig River, btw - with rail connections to other nearby historic towns (Gutach and the Vogtsbuauernhof, Schiltach, Villingen for example) in the Black Forest as well as to Strasbourg FR and to Paris (Offenburg, just minutes from Gengenbach, is the transfer station.)
Get to Baden-Baden, Donaueschingen (source of the Danube River) and Lake Constance on the Black Forest Railway, one of Germany's most scenic train routes.
Day-trip to Freiburg (1 hr.) for a visit there, or to Heidelberg (2 hrs.) on the Rhine Valley Railway.