User blog comment:ImperiexSeed/The oldest being, archangel or leviathan?/@comment-25291610-20140626230522/@comment-3045936-20141029171331

It was 30-40 million souls in Castiel. And despite that he barely could get the Leviathans inside to stay in check, after awhile. So it leaves to interpretations that maybe an Angel with enough power can affect Leviathans. But it is confirmed they are older than Archangels.