Susan only appears in a pre-filmed insert in order to give Carole Ann Ford a holiday during production. It was recorded during production of Sentence of Death (1964).
This episode takes place in Mexico in the 15th century.
This was the first of three serials that Walter Randall would have roles in during the William Hartnell era. The others were "The Crusade" and "The Daleks' Master Plan".
This episode was watched by 7.4 million viewers on its original transmission.