Duke's Rare Book, Manuscript and Special Collections Library has excellent resources in architecture, including folio volumes of plans. Because of the nature of its collections, its riches are not as discernable through the Duke catalog as other material. In addition to doing a search limited to Rubenstein Library, email or query a special collections librarian for a consult on what may be of interest. RUBENSTEIN SEARCH
For example, type in ARCHITECTURE GERMANY in subject field.
WHY BOTHER DOING ARCHITECTURE RESEARCH RIGHT? (No, seriously) Four quick videos
Medieval Studies at Duke https://guides.library.duke.edu/medren
European Studies at Duke https://guides.library.duke.edu/subject/europe