Jane Asher asked Roger Corman if a friend could visit the set and join them for lunch. She explained that her friend was a musician who was about to do his first gig in London that night. At the end of lunch, Corman wished him good luck with his concert. Corman had never heard of Paul McCartney until he read of the concert's success in the next day's newspapers.
Hop-Toad and Esmeralda were drawn from Edgar Allan Poe's short story "Hop-Frog". That story was inspired by an actual incident in 1393, where French King Charles VI and 5 of his lords dressed and chained themselves as "wild men" at a masquerade, and their costumes caught fire from a stray spark. Four of the lords died, and the incident became known as the Bal des Ardents (Ball of the Burning Men).
Roger Corman originally wanted this to be his second Edgar Allan Poe picture following the success of House of Usher (1960). He passed it over because he felt that certain plot elements were too close to Ingmar Bergman's recent The Seventh Seal (1957).
Roger Corman shot this film in England to take advantage of British tax laws. By making this a British production with a British crew, the production got a government subsidy.
Roger Corman and Daniel Haller were able to make this film look more opulent than earlier productions by using the sets left from Becket (1964). The two movies also share an actor, David Weston, who plays Gino here, played Brother John in Becket.