One of the characters reads a letter from Hamlet where he describes being captured by pirates and the action in the letter is depicted in a blurry framed vignette.
There is a credit for three special effects men but according to IMDB the uncredited visual effects personnel included George Blackwell and Bill Warrington both well known for their miniature work along with James Snow who was well known for miniature pyrotechnics.
The models were shot in a tank at Denham Studios.
