I don't think that Castlevania is really supposed to be scary. It's an action game. And platformers tend to be action games, not horror/suspense games, because navigating platforms is inherently an action-ey concept. But that's not the question, so I'll drop it.
Atmosphere is absolutely key. Having things like fog or interesting lighting effects (low lighting, flickering light, moving light sources, etc.) and subtle music, if any music at all, will contribute to that atmosphere. Color schemes need to be appropriately muted or realistic, and sound effects should be impeccably placed. All that jazz.