Without having seen the details, I think the combination of cities and player power is a lot better than just the amount of cities.
(To open a window you need to drop cities AND get the troops of many players injured)
What I strongly dislike is, that you can still only not join the strongest kingdom, which will lead to only 2 kingdoms left sooner or later.
You could change this very easily and lock the strongest 2 if not 3 kingdoms.
This way, there would be a lot more dynamic in the endgame-play, since a 1:1 fight seems boring.
I would also strongly suggest to not allow the strongest kingdom to create or accept any alliances with other kingdoms.
Both changes combined would make this game a lot more fun (pvp wise)!