User blog comment:Zippertrain85/Dawnguard Follow up./@comment-17114085-20131216013719/@comment-17114085-20131223013453

1) Who knows about them?, no one has ever mentioned the Volhikar Clan in specific, and the Dawnguard were unaware of the Castle's existence until the Dragonborn told them about it.

What do you mean Zip? Yes it was colonized but picture it like the Thirteen Colonies of the USA. Each had a governor, and no president, and all were vassals to the King of England. Same with Skyrim, there was different Holds, all with different leaders(Ysgramor probably ruled both Eastmarch and Whiterun). Harkon could still have been one of those leaders, but again this is all speculation on our part.

I do kind of like Ralok's explanation about Harkon ruling the Coldharbor version of Skyrim, though that would still leave us with the fact that after the Interregnum he would have to have left Coldharbor and travelled to Skyrim, found the Castle, lived there for a long time, and then Serana would have to have been trapped in the ruin for a long time. Which would have to have been enough time for the ruins to look different as Serana complains they look different then when she was buried there,