Prussian Privy State Archives

The Prussian Privy State Archives (German: Geheimes Staatsarchiv Preußischer Kulturbesitz or GStA PK) is an agency of the Prussian Cultural Heritage Foundation headquartered in Berlin, Germany. A Federal statutory body, it is one of the largest repositories of Primary source documents in Germany and spans the history of Prussia, Brandenburg, the House of Hohenzollern and the Prussian Army.

More information on the Wikipedia page.

The website, in German, is.