Orson Welles often said that making this film was the most sheer fun he had ever had, working in the cinema.
While the latter portion of the plot revolves around the diamond necklace, the real history is different. In 1772, the infatuated Louis XV requested that Parisian jewelers Boehmer & Bassenge create an elaborate and spectacular jeweled necklace for Du Barry, one that would surpass all known others in grandeur, at an estimated cost of two-million livres. The necklace, still not completed nor paid for when Louis XV died, would eventually trigger a scandal involving Jeanne de la Motte-Valois, in which Queen Marie Antoinette would be wrongly accused of bribing the Cardinal de Rohan, Archbishop of Strasbourg in the Alsace, to purchase it for her, accusations which would figure prominently in the onset of the French Revolution.
Initially, the film was to be directed in the USA by Douglas Sirk and was to star George Sanders.
Orson Welles would star with and direct Akim Tamiroff many times after this picture, most famously in Touch of Evil.
Superman #62 (Jan-Feb 1950) has a story in which Orson Welles is kidnapped by Martians while making this film. Nancy Guild is also in this story.
The actors who portray Cagliostro, Orson Welles and Annielo Mele, have repeatedly worked with Vittorio De Sica: Welles acted with De Sica in The Battle of Austerlitz (1960), Lafayette (1962) and 12 + 1 (1969) whereas Mele acted with De Sica in Lost in the Dark (1947) and appeared under De Sica's direction in Shoeshine (1946).
The Masonic compass that Cagliostro sported on his hat and other "secret-fraternal" symbols on his coat when being presented at court are eloquent. He was anti-monarch (although he coveted the power for himself), and could easily have been a member of one of those secret societies.