Although the publicity for this movie made much of the fact that the three greatest horror stars of the day, Vincent Price, Sir Christopher Lee, and Peter Cushing were all in it, the three actors have in fact only small roles in this movie, despite star billing. Cushing's role is confined to one scene, without the other two, and the Price and Lee characters have a very brief scene together only at the end of the movie. The combined footage for all three actors only comes to about one-fifth of the total running time.
Several years after this movie was made Vincent Price said in interviews he never understood the script at all.
The original novel featured aliens as the villains but they were replaced with mock-Nazis in the movie version. Instead of a Swastika, they have three connected up-arrows on a white circle on a red armband.
Filming began May 5, 1969. The only other movie to team Vincent Price, Sir Christopher Lee, and Peter Cushing was "A Mansão da Meia-Noite (1983)."