The film is the fourth and last of Hammer's "Cave Girl" sequence of films, preceded by One Million Years B.C. (1966) (also directed by Don Chaffey), Slave Girls (1967) and When Dinosaurs Ruled the Earth (1970).
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This process was also employed by Jim Danforth to blur the pterodactyl's wings in Hammer Films' "When Dinosaurs Ruled the Earth", and by Randal William Cook on the terror dogs sequence in "Ghostbusters".