First and foremost, in my opinion, Castlevania is known for its music. The track "Bloody Tears" were remixed and remastered countless times officially and by fans. The worldwide Video Games Live concert tours would always have a Castlevania medley in their playlist.
Secondly, Castlevania is known for it unique lineup of heroes; The Vampire Killer wielding Belmont clan, Alucard, and Soma Cruz.
And then, there is the platforming and infamous Medusa heads that brings players to their untimely deaths on all the 2D games.
Yep, gotta love those Medusa heads.
And last, but not least, Dracula returning for every game. He has got to being one of the most active villains besides Bowser from Super Mario Brothers. Every time something goes wrong in the Castlevania universe, it's always Dracula or someone trying to resurrect him.
Pretty much, without Dracula, there will be no Castlevania, period.