Reviewers seem to dislike the ending, but I think it's fun. It does turn the movie on its head, but I think it's a fine way to wrap it all up.
Moria says, "What finally does House of the Long Shadows in is its escalating series of multiple twist endings that reduce it to absurdity." British Horror Films says,
Suffice to say that 90 minutes of Cushing, Lee, Price, some bloke called Carradine, some terrific gory deaths, tons of atmosphere and a gripping storyline are wiped from the memory by what can only be described as a sub-Murder She Wrote finale. You have been warned.