Each of the seven tourists, represents one of the Seven Deadly Sins.
This film was shot in a real castle in Belgium.
Screenwriter Patrice Rondard, who frequently used the pseudonym Patrice Rhomm for his literary and cinema industry work, also wrote a novelization entitled "Au service du Diable". The 216-page French-language paperback was published by Galliera in April 1971 to coincide with the film's European theatrical release. Galliera issued the book as the first volume in its "Bibliothèque de l'étrange" (Strange Library) series. It features a full-color illustration on the paper cover depicting Erika Blanc and Daniel Emilfork as their respective characters that is moderately different from the artwork used for the theatrical release poster.
The "dead kitten" was actually sedated, as it is clearly breathing.