When the Shadow rolls a bomb toward the detectives in the secret passage, Dr. Tart says, "We must be near a bowling alley." He's right: there is indeed a bowling alley inside the Biltmore House in Asheville, North Carolina, where this movie was filmed.
During the shoot a priceless book went missing from the library in the mansion and Tim Conway was accused of stealing it. In January of 1981, former security guard Robert Livingston Matters was tried and pleaded guilty to stealing 60 rare books from the 25,000 volume library. Tim Conway was called the key to the theft for bringing attention to the missing books. As an employee for the manor went to show him a rare book and realized it was missing, that's what spurred the investigation.
The secret passages featured in the film actually exist in the Biltmore mansion. The only portion of the passage that was constructed is the rotating wall, and the owner of the house liked it so much that he decided to keep it.
According to the book "Mind Warp!: The Fantastic True Story of Roger Corman's New World Pictures" by Christopher T. Koetting, this picture was New World's most successful film at box office under Corman's tenure.
The name of the hairy monster in the film was a "Wookalar". It was defined as being manlike creature with a pig-like face and superhuman strength. Moreover, The character of the caped figure was billed in the credits as being "The Shadow".
The characters of Inspector Winship (Don Knotts) and Dr. Tart (Tim Conway) were a parody of Arthur Conan Doyle's Sherlock Holmes and Dr. Watson respectively. Winship wears the trademark Sherlock deerstalker hat but smokes a cigar instead of a pipe. Both however are detectives for an official law enforcement body whereas Holmes and Watson were both in the private sector. The Tart character is also a doctor like Watson.
At the beginning of the movie, Lord Morley says "All's well that ends satisfactorily," thus mixing up the phrase. This is a subtle clue that he is The Shadow, since The Shadow destroys his own rhymes by mixing up phrases.