ODH is a seamless blend of comedy and old fashioned murder mystery. Most of the film takes place in Femm Manor, an old dark house, which is inhabited by an eccentric British family. Unfortunately for them, they are being killed off one by one under strange circumstances. At the center of the story is Tom Penderel, an American, who was invited to stay at the manor by his friend Caspar Femm who was the first of the family to meet his demise.
William Castle simply knows how to direct this kind of film to perfection. With a clever script from the hands of Robert Dillon [13 Frightened Girls, French Connection II], Castle is off and running, knowing instinctively how to make the most of every clever line. His sense of timing is impeccable and I'm sure 1963 audiences loved this film.
The other reason why this film works so well is its first-rate cast. Tom Poston [The Bob Newhart Show] plays Penderel with great expression and frenetic energy. He's perfect for this film. He is also surrounded by a cast of veteran British actors who know how to make this kind of dialogue work. Particularly memorable are Robert Morley [The African Queen, Beat the Devil] who would give the dowager countess from Downton Abbey a run for her money. Fenella Fielding is comedic perfection as Morgana the sexy sister who has the hots for Pendrel.
Don't let the rating on IMDB fool you. The Old Dark House is a thoroughly enjoyable and entertaining film.
For more info check out the film's entry in IMDB.