Franco Nero's voice is dubbed. Cole is from Texas, and Nero couldn't master an American or Texas accent.
After Mike Stone was let go because of his lack of acting ability, Director Menahem Golan found out that Franco Nero was in Manila and quickly approached him for the role and Nero agreed to do the film. Stone stayed on the film as Nero's stunt double and it is Stone who handles the majority of the action sequences as the ninja.
This is the first of three "Ninja" movies in Cannon's Ninja series. The others are Die Rückkehr der Ninja (1983) and Die Herrschaft der Ninja (1984). Both sequels were unrelated in terms of story. Cannon also went on to make American Fighter (1985), which spawned its own five-movie series.
Franco Nero had no martial arts training, so he was doubled by writer and fight choreographer Mike Stone for the fight sequences.
This is widely considered the movie that popularized Ninjutsu during the 1980s. It was actually the second major English-language Ninja film of the 1980s. The first was Octagon - Die Rache der Ninja (1980).