Certainly one of the best movies directed by Mario Bava.
The atmosphere is frightening. The photography is splendid (as usual for a Mario Bava's movie) and this movie contains some close-to-experimental scenes that deliver a weird but delicious feeling.
Furthermore this movie is certainly an inspiration for many famous filmmakers. You may think of Tim Burton (Sleepy Hollow), David Lynch (Twin Peaks), Guillermo Del Toro (Crimson Peak) and even Federico Fellini (Toby Dammit) while watching this movie.