Before Richard Widmark was cast, his role had been offered to Klaus Kinski. He turned it down, stating that he had no issue being in a film where his young daughter was fully naked, but he couldn't guarantee he would remain sober for the duration of filming.
This was Christopher Lee's last Hammer film until The Resident (2011). So it was Christopher Lee's last Hammer Film of the original classic incarnation for which the studio is still famous, and he was one of the major players who made the studio legendary.
This was Hammer's final horror film. Although some call it Hammer's final production, they did release "The Lady Vanishes" (1979) three years later and then ceased production. Hammer would be resurrected and resume production in 2008.
Richard Widmark soon realized that this was a troubled production and had to be talked out of quitting on several occasions.
The notorious Sir Christopher Lee naked scene was actually performed by his regular stunt double Eddie Powell, who was filmed from above, in order not to show his face.