The roar of the dragon guarding the Golden Fleece is actually Godzilla'a roar from Godzilla - Das Original (1954) (U.S. title: "Godzilla, King of the Monsters"). Joseph E. Levine had acquired and released that film three years earlier.
During the scene in which Hercules uses chains against enemy soldiers, director Pietro Francisci became annoyed with Steve Reeves because he felt that Reeves wasn't swinging the chains hard enough to be convincing. Reeves claimed that he didn't want to swing the wooden chains (painted to look like they were metal) too hard because he didn't want to hurt the actors. In response, Francisci shouted back "If they don't get hurt, they don't get paid!"
Joseph E. Levine acquired the U.S. distribution rights to this film, which he retitled "Hercules," for $120,000. Levine called it, "One of the worst pictures I ever saw, but I knew it had great appeal. There was a market for anything then." Levine publicized it on TV and it grossed a million dollars in its first ten days.
This is the film that started the craze for movies about muscle-bound mythological heroes. By the time the genre had run its course in the mid-1960s, 170 such films had been produced in Italy. This represented approximately 10% of Italian film production during this period.
The film's cinematographer Mario Bava claimed credit for suggesting that Reeves grow a beard for the role.