The film slipped into the public domain not long after its theatrical run was over. That explains why so many poor-quality versions of the film are available.
In his autobiography "The Actor's Life - Journals 1956-1976", Charlton Heston wrote, "We're faced with the endless problems of organization, personnel, dogs, publicity . . . I fear I've fallen in with amateurs and con men. This had not been a picture really but a production deal, patched together with incredible adroitness and negotiating skill--and no filmmaking talent whatsoever."
Juan Luis Galiardo had a nervous breakdown while acting in this film and was unable to finish his part. Sancho García replaced him, playing a different character in Galliardo's unfinished scenes.
Charlton Heston had director approval on the film, an unusual concession on the part of notoriously parsimonious producer Harry Alan Towers.
Charlton Heston had to use real bullets in the scene where he shot at the wolves, shooting over their heads, because the crew forgot to get blanks.