Obviously I don't actually expect a definite answer to this, but I noticed a few times throughout Dawnguard's questline that Serana would mention whoever was in the castle before she and her parents moved in (mentioning the jetty, Valerica replacing the sundial with a moondial, etc). I dunno if it's anything resembling a clue, but I did notice a partially destroyed statue to Mara in part of the ruins, which seems like a weird thing for a bunch of vampires to have, especially Harkon.

But just thought it might be an interesting thread to toss ideas around. What do you think?

IIRC there was mention elsewhere that Castle Volkihar was some manner of convent to Mara before the vampires did what they do best to it.

Really? Huh, seems like an awfully big ass castle for Mara worshippers, will have to see if I can find that reference somewhere.

Interesting.

There isn't a definite answer to that but as Madcat said there were Mara worshipers in the castle. At some point you pass by the statue of Mara.