Another random thought on the castle issues, plus incentivizing people to play together in one province. What if the castle count for the entire province gave scaling production bonuses to the members of the province. This would both deter people from sitting at lower castles to "play it safe", and encourage people that are playing together to be in one province vs scattered across random ones so they can hit each other for protection. Would also cause people to think less about their individual kingdom and more about the province as a whole, something that I think is currently lacking - getting an extra castle exposes your kingdom slightly more, but all of your province mates get the benefits of extra resource production. Players would be less likely to give up mid round, as their kingdom performance is now more tied in with the rest of the province.
Thoughts?