Despite being present from the opening, and also top billed, Jack Palance (Ronald Wyatt) does not speak until one hour and fifteen minutes into this movie.
When this movie was shown in 1967, one of the promos was a pack of torture garden seeds given to patrons as they entered the theater. The seeds were actually grass seeds.
"Columbia Pictures" financed this movie and insisted that two American actors be cast. Therefore, the initial casting of Peter Cushing (Lancelot Canning) and Sir Christopher Lee was changed to that of Jack Palance (Ronald Wyatt) and Burgess Meredith (Dr. Diabolo).
A scene featuring Burgess Meredith is used at the beginning of the film "See No Evil" (1971) on a TV set in a shop window.
This movie was released in the U.S. as the top half of a double feature with "Trog (1970)," which was Joan Crawford's final movie. Both movies were directed by Freddie Francis.