I am sure many of you already know this, but i just realized it and maybe there are others like me who will find this information useful:
Anything in cyrodiil that is a FORT naturally belongs to daggerfall covenant.
Anything in cyrodiil that is a KEEP naturally belongs to ebonheart pact.
Anything in cyrodiil that is a CASTLE naturally belongs to aldmeri dominion.
This distinction is important for the keep-held bonuses (enemy structure held gives a different bonus than home structure held)