Boris Karloff became sick with pneumonia while shooting this project in the freezing rain. It was his last British movie. Filming began on January 22, 1968. Karloff had just finished "Targets (1968)," and would recover enough to shoot four Mexican features in May 1968, his final screen work.
Christopher Lee admitted to only accepting his part in the film, after learning about Boris Karloff's decaying health and viewing this film as a final chance to work with him for a last time.
Boris Karloff had to remain in a wheelchair for the majority of his screen time, due to his having arthritis.