As was common in German films of the era, this movie was shot without live sound, and all voices were dubbed in post-production. Christopher Lee and Thorley Walters were not scheduled to dub their own voices for the English dubbed version (or any other), and were dubbed by other actors.
While shooting parts of this German film in Dublin, some gypsy extras came up to Christopher Lee between takes and inquired about his origins. From the observation of his physical type and the few informations Lee told them about his father, their conclusion was that he must definitely have been of some gypsy ancestry himself. Knowing very little about his father's family, Lee gladly welcomed this assumption as a very believable one.
When Christopher Lee and Leon Askin were filming the scenes down the sewer seen near the end of the picture, they found the smell to be 'a bit stiff, even for a sewer'. After some inquiries, they found out the place had been used during World War II to make poison gas.