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Dear Mr. Keim,
Thank you for posting your request on History Hub!
We recommend that you contact that current owners of the castle as they may have an archives with relevant documents or know which repository has custody of the records. You may also wish to contact the European Castles Institute as well as national, regional and local archives, libraries, and historical societies for the area where the castle is located. FamilySearch has articles on Germany Archives and Libraries and France Archive and Libraries that may be helpful.
We hope this information is helpful. Best of luck with your research!