One of Castle's earliest successes, 13 Ghosts told the story of Buck Zorba, a young boy who used a pair of "ghost goggles" to navigate through a haunted house stocked with an assortment of spirits that included a murderous chef, a burning skeleton, a ghostly lion, and other challenges suitable for a party of four third-level adventurers. In order to draw the audiences into the film -- or at least get them into the theater with a paid admission -- Castle gave viewers their own "Ghost Viewer" and even appeared in the film with his typical charm to explain how it was used.
The film used the same sort of red overlay that was used for 3D pictures, with the difference being that you were only supposed to look through one lens at a time. With the red lens, you could join Buck in seeing the ghosts. With the blue lens (the "ghost remover"), they were rendered invisible, which Castle promised would prevent heart attacks among his more sensitive audience members.
Here's what makes it genius: It's completely pointless. Not only were the "ghosts" clearly visible without the use of the red lens, nobody who doesn't want to see a ghost is going to go into a theater to see a movie called 13 Ghosts. There is no reason at all to have a little cardboard and cellophane "ghost viewer" thrown in, but as you might imagine, the kids loved it.