Originally conceived as a Beatles vehicle by veteran Beatle director Richard Lester.
The Duke of Buckingham's billiard room and the long hall behind it at his English estate is a leftover set from Patton (1970), it is also seen in The Four Musketeers (1974), where Milady and Buckingham speak about American Indians and also serves as a set of rooms in The Return of the Musketeers (1989) for a different character in France some 10 years after the action of the first two films. Also, the gown worn by Faye Dunaway during the Ball sequence at the end, is also the main gown worn by Jenny Agutter in The Man in the Iron Mask (1977) (TV).
Originally the director was to be Tony Richardson, but he was discarded after he said that the entire movie should take place in a convent.
Various sources including Charlton Heston's memoirs say that Heston was offered one of the Musketeers - the Oliver Reed or Frank Finlay part - but Heston passed on this role or roles because it was too demanding - he had just done a run of action roles - and also because the part was a bit small, so he chose to play a cameo.