The problem with that is that you run out of provinces way too quickly
Good point. Hopefully they have increased the limit. If not, the idea could be used for smaller campaigns, eg, English Civil war, 30 years war, 80 years war etc.

If it could be set so that the castles drew from the city province, then all right.
That would be good, there could be a button in the castle that transfers X number of people from the city to the castle.

We will have to wait to find out.