For me it is the theme to the Dario Argento classic Phenomena by Goblin. The music just screams Castlevania to me. Also if you've not seen Phenomena I highly recommend it. It's a great gateway movie into the weird wonderful world of Italian horror/giallos added bonus it stars a young Jennifer Connelly as a teen who has the ability to psychically control insects. Did I mention the movie also contains Donald Pleasance of Halloween fame and a scalpel wielding chimpanzee that IRL bit off part of one of Jennifer Connelly's fingers?