I don't think it's fair to say that CK2 has been placed into the Civilisation series when it comes to Old World. There is only ever one title the player can have (King/Queen of your nation) and though the character interaction in OW is great (they grow, get an education, acquire traits and you can nudge them in the right direction and improve them), it's not quite as detailed as CK. In terms of complexity, it's definitely closer to Civ IV level of design and interaction than the later Civ games. It has a lot of moving parts and it can take many playthroughs before you feel comfortable with everything, let alone learn to use mechanics to their full potential. It definitely feels like a spiritual successor to the older Civ games, which makes me happy.