• We have updated our Community Code of Conduct. Please read through the new rules for the forum that are an integral part of Paradox Interactive’s User Agreement.
Even though i have no idea about modding, all i did was (after making a backup) change the primary settlement of the county in the provinces.txt.

So, just change it to "castle" and you should be fine. make sure you still have one of each kind when you start the game, so just make the primary one a castle, and the former castle a bishopric or city.