The opening credits feature the camera floating through nine sets of doors before the film begins.
The two booksellers Corso encounters in Toledo are actually the same actor, José López Rodero. Writer/director/producer Roman Polanski used a motion capture rig to use the same actor twice. The same man appears again later, playing two workmen cleaning out the bookstore. Rodero was an assistant director and production manager, not a professional actor. He was hesitant to accept these multiple parts.
The voice of one of the Ceniza twins (Pedro Ceniza) is dubbed by Roman Polanski himself.
The pen that Lucas Corso uses is a limited edition Montblanc Agatha Christie ballpoint.
The keypad combination to Boris Balkan's penthouse office and to his private library is "666".
The book of the nine gates, they say was written in 1666.
The little girl that stares at Corso at the Paris Airport is played by Roman Polanski's daughter Morgan.
The film has a different title from its source novel, "The Club Dumas", because the novel's title referred to a major subplot involving a secret society obsessed with serialized novels, such as those written by Alexandre Dumas père.
The cigarettes that Liana Telfer smokes are called Black Devils.
The car that Corso and 'The Girl' drive in France is a Chrysler Dodge Viper. The brand logo of Chrysler is a pentagram, the model name is Viper (which refers to the snake from Adam and Eve) and the car itself is painted red, the color of the devil.
Chateau Puivert (the Ninth Gate) is called The Devil's Tower. It was used by the Cathars to defend themselves during the Albigensian crusades in the 13th century. The Cathars were gnostic Christians but Pope Innocent III considered them to be devil worshippers.