This film and the other three horror films that were all in the same package were originally supposed to be filmed entirely in Mexico City, Mexico in 1968. At that time, however,
Boris Karloff was 81 years old and in extremely poor health, was suffering from both advanced emphysema and rheumatoid arthritis, only had one-half of one lung that was still functioning (both the other half of it and all of the other lung had already been removed due to lung cancer (Karloff had been a heavy smoker for most of his adult life)) and could only breathe through an oxygen mask that was connected to a mobile oxygen unit. In addition to all of this, his doctors had already told him not to travel to Mexico City because of the thin air at its high altitude. As a final result, all of Karloff's scenes for all four of these films were filmed on a soundstage in Hollywood in 1968 out of necessity.
