It certainly has all the ingredients to make this a classic, but fails to make it through for me, despite the pretty impressive cast.
It does bring to mind the Deborah Kerr cult movie The Innocents, but doesn't quite get there. All the components are here, but I thought the lack of pacing of the film made this less effective entertainment.
It is interesting nonetheless if you can take the slower pace. The outside shots mostly take advantage of the set location in France in a castle or chateau, which adds to the atmosphere quite well. The film does have the feel though of being filmed the earlier 1960's, rather than in the second half, with the camera angles etc i.e. close up of the actor to the side of the shot with the action/plot developing in the distance. It's shot in black and white which works very well, just at this time in cinema, there was the colour explosion going on.
The two "youngsters" here, David Hemmings (died blond hair) and Sharon Tate certainly look good, which is handy, as they do not appear to have many lines. They mostly go for the quiet manacing look school of acting here.
For those who like a bit of S&M, Sharon Tate gets a whipping and seems to like it, but don't expect too much, and that includes any erotic scenes or nudity. Just as well, as they are not needed, although it may have made the film slightly more interesting if tastefully done.
Sharon goes through the film looking like a model here in nearly all of her screen time. Donald Pleasance is Donald Pleasance who gives most of his best acting with his eyes, and does not have the lines or role to make his performance more memorable, which is a shame. As to David Niven's performance, it was good enough, but no particular plaudits from me (or criticism). Deborah Kerr is quite good, and is trying reasonably hard I thought, as did Flora Robson.
It's a 60's, B/W film, driven by a pagan plot with the usual accoutrement's (monks in hoods etc), and if that is your thing then give it a go. Not an out and out classic, but is worthy of being mentioned with the best of this genre, and more as an interesting flawed cult movie for horror fans.
