During filming, Scott Adkins injured his back and required stunt double Brahim Chab to perform some of his kicks for him. This marks the first time that Adkins had been significantly doubled for a fight scene.
While shooting the torture scene, Scott Adkins was so sweaty that drops of sweat started to falling on the floor from his shirtless body. After an hour there was a sweaty puddle under the chair Adkins was tied up to, and the set had to be cleaned.
The fight scene with Ron Smoorenburg in the dojo is one single take, which makes it even more impressive.
The knife that shown in the fight after Casey buys the pendant is a Japanese ceremonial knife called a tanto.
Near the end of the film goro calls casey a Gaijin but the subtitles say kaiju, outsider or a giant monster