J. Carrol Naish was very old and frail at the time that this film was made and, as a result, he could no longer remember dialogue, so he read his lines in it off of cue cards. However, he had only one working eye; the other one had been replaced with a glass eye long ago. In Naish's close-ups in the film with dialogue, one eye can be seen moving back and forth when he is reading his lines, while the other eye does not move at all.
Much of the electrical equipment in Dr. Durea / Dr. Frankenstein's (J. Carrol Naish) laboratory beneath the Creature Emporium boardwalk attraction was originally used in the classic Universal films Frankenstein (1931) and Bride of Frankenstein (1935). Ken Strickfaden, who had designed all of the aforementioned equipment that was used in those two films, supplied it for this film.
When Dr. Durea / Dr. Frankenstein (J. Carrol Naish) confronts Count Dracula (Zandor Vorkov) in the Creature Emporium boardwalk attraction after it is closed for the night, Naish looks older than he does elsewhere in the film. Over one year had elapsed between the filming of most of Naish's scenes (when it was intended to be a different horror film altogether) and the Count Dracula / Frankenstein Monster scenes that were grafted into the film.
J. Carrol Naish and Lon Chaney Jr. completed their original scenes for the film between March and April of 1969, and the Count Dracula / Frankenstein Monster scenes for it were then completed over one year later.