Boris Karloff became ill with pneumonia while shooting this project in the freezing rain. It was his last British film. Filming began on January 22, 1968. Karloff had just finished making Targets (1968) and would recover enough from his illness to star in four Mexican horror films that were all released in May of 1968, his final screen appearances before his death on February 2, 1969.
Christopher Lee admitted to only accepting his part in the film after learning about his friend Boris Karloff's frail health and seeing this as a chance to work with him one last time.
Boris Karloff had to act in a wheelchair for the majority of his screen time in the film due to his having arthritis.