So I am playing my first game of BtS and I am a long way though it (about 1970's) but from about the 1600's on I keep getting a messages every few turns that a city of mine want to join another country!!! And not just any other country either, they want to join my Vassall!!! Why is this, I have a much higher culture score than them and I am starting to wonder why none of their cities are joining me!!! Of course I won't allow this to happen, I am not just going to start handing my cities over to others! Has anyone else experienced this?