The TV show that Harrington refers to in an attempt to fool Inspector Russell is a clip from Mario Bava's own I tre volti della paura (1963) - specifically the "Wurdalak" sequence featuring Boris Karloff.
Originally the script didn't include the character of Mildred Harrington. It was only after Laura Betti expressed interest in working with Bava that the director re-wrote the script so that Betti could have a suitable starring role.
The Spanish villa that the majority of the film was shot at was formerly the home of Spanish dictator Generalissimo Francisco Franco.
Laura Betti and Dagmar Lassander didn't get along very well, with Betti being jealous of Lassander and treating her very derogatory.