This was going to be Roman Polanski's first American film. Robert Evans of Paramount needed someone to direct Rosemary's Baby (1968) so Polanski was given that project due to the nature of some of his recent films. Robert Redford was also considered for the male lead in that film.
Sylvester Stallone was an extra in this movie. He shares a scene with Robert Redford and Camilla Sparv. 7 years later, the producers of Rocky (1976) wanted Robert Redford cast in the role of the film's title character, Rocky Balboa, but Sylvester Stallone was determined he wanted to star in it. He did, and he became one of Hollywood's biggest-ever stars.
Natalie Wood's then husband, Richard Gregson, was one of the producers on this film. She was also a good friend and former high school classmate of the star, Robert Redford. She provided hair styles for some of the actors, provided wardrobe styles for some of the stars, and was a visible presence on the European locations for the film.
Edith Head had begun work on this film back in the spring of 1967 when her contract with Paramount can to an end. It is debatable how much of her contribution still remains in the completed film but she regarded this film as one of her 1000+ films of her incredibly long costume career.
Star Robert Redford, in the commentary on the DVD, reports that this film was in pre-production for a long time. He was supposed to appear in Rosemary's Baby and then this pet project of his was to go before the camera. Due to a breakdown in negotiations and his eventual refusal to appear in Rosemary's Baby, this film was canceled at least twice before Redford's production company picked up the film and began moving forward on the project.