Trivia

This is one of the few European movies that Sir Christopher Lee made without providing his voice for any of its language tracks, as he had returned home to Switzerland by the time shooting wrapped (as a result, he is dubbed in the English version by William Kiehl). He would quickly regret not dubbing his voice for this movie, as he considered it to be the best of his European movies, and ensured that he provided his voice for at least the English versions of all of of his later movies.

Most of the cast and crew were required to use English pseudonyms because the producers hoped to fool the intended Italian audience into thinking this movie was produced incognito by a British or American studio, such as Hammer Film Productions or American International Pictures. When Mario Bava was asked by Luciano Martino to use "an old American name", he jokingly took the suggestion to a literal degree by creating his alias "John M. Old".

Sir Christopher Lee had hoped to work with Director Mario Bava on another movie, but their busy schedules kept them from working together again. Lee had also heard inaccurate rumors suggesting that Bava's mental health was in decline, and upon seeing Bahía de sangre (1971), he was so disgusted by its violence that he left the theater in protest.