The huge fortress of El Condor, built in Spain for this movie, was subsequently featured in many other films, including A Reason to Live, a Reason to Die (1972) and Conan the Barbarian (1982).
The producers had built the 'El Condor' fort, then found they weren't happy with the original script, so had Larry Cohen re-write it to have the story focus more on the impressive set.
Ewa Aulin was the first choice for the Marianna Hill role, but director John Guillermin got off to a very bad start with her. He reportedly began by asking her to strip naked so that he could see if she was qualified for the part since it involved nudity. Aulin, who had stripped willingly enough in Candy (1968), objected strenuously. So did her husband. The way Aulin remembers it, Guillermin made matters worse by saying he could not understand her modesty and telling her, "You're no better than a whore." Bystanders kept her husband from Guillermin's throat, and Ewa dropped out.
El Condor was among the first movies rated R (for violence, explicit language, and nudity).
Marianna Hill didn't want to take the role because she'd heard stories about other recent films made in Spain. But her manager talked her into it. She said later it ended up being a bad experience, though she admitted she didn't remember much about it. "I was confused all of the time, as everybody was. And I don't drink, I never touch the stuff, but there was a lot of drinking going on down there and so that affected the Karma on the set. I just couldn't figure out what was going on." She often refuses to talk about the film in interviews.