Fred Williamson was already in Italy completing work on another film. When he found out about this film he tracked down director David Worth and asked him to give him a role in the film just so his work visa could be extended allowing him to stay in Italy longer because he enjoyed the country so much.
Riffed by Joel and the Bots from Mystery Science Theater 3000, season 5, episode 1.
David Worth explained in an interview that the story the poster existed before the movie is "apocryphal." This interview is available in the DVD extras for WotLW release as part of MST3K Volume 26 (XVI). In an on-camera interview Mr. Worth details how he was introduced to the producers and instructed to "make the movie" based wholly on his 40 page treatment.
Donald Pleasence's footage was filmed within 3 days.
During the filming of the scene where Prossor interrogates Nastasia, Donald Pleasence kept encouraging Persis Khambatta to spit in his face more with each take as he felt it was giving his character motivation.