Enjoyment of this film will depend on two things. First how much you like the actors involved. The second is the understand that this film was pieced together from scraps of film shot by various directors to fit in with footage that had been shot of Karloff on left over sets after the original feature had been finished. If you can forgive your favorite actors anything and accept that this is a paste up job, then you'll have yourself an okay time at the movies.
Lets face it this film is a mess. The story of a French soldier following a mysterious woman to a castle is so disjointed that they actually shot a scene where Jack Nicholson grabs one of the characters and asks him whats going on, its at that point all of the loose ends are brought together in a tenuous grip.
Its the sort of movie that shouldn't work on any level but some how does.Its well acted considering that no one probably had any idea what they were starring in. Its also directed well enough that you don't realize that there were actually five directors other than Roger Corman behind the camera.
Currently floating around in the public domain (cheap video copies are to be had) this is a movie worth seeing if you want to see how Jack Nicholson started or how Boris Karloff ended up. Its a just okay thriller with a more interesting production history. Worth a bag of popcorn if you need another movie to fill out a night of Corman Poe movies.