Germans like to travel, there are 81 million of us and our Nr.1 destination is Germany. And the destination Nr.1 in Germany is Bavaria, so yeah, it is pretty much discovered.
if you like castles google the biggest ones and just visit them.
If you like to wander I recommend Rhineland-Palatina and there the "Dahner Felsenland", they even have a rout called "castle way", "Burgen Weg", here a overview:https://www.tourenplaner-rheinland-pfalz.de/mobile/de/tour/praedikats-kurztour-praedikats-rundtour/deutsch-franzoesischer-burgenweg/6829321/
If you come during the fall you should visit the wine festivals, great stuff.
Another thing you might look into is "Sächsische Schweiz", also great nature but less about old buildings.
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