There should not be fixed territorys and procinces but rather territorys defined by occupation. Lets say Dejure county/province of essex can be occupied by different realms by sphere of influence and army occupation rather than fixed borders.
Territorys/provinces should have more use than just be place for few castles. They should be farmlands or places for estates for minor nobles/gentry. And lords should be able to build outposts to control territorys in time and development they could grow to be citys or castles/fortresses what ever lord design them to be. In time half occupied essex could become dejure part of your realm similar as ck2 duchy becomes part of other kingdom.
Territorys/provinces should have more use than just be place for few castles. They should be farmlands or places for estates for minor nobles/gentry. And lords should be able to build outposts to control territorys in time and development they could grow to be citys or castles/fortresses what ever lord design them to be. In time half occupied essex could become dejure part of your realm similar as ck2 duchy becomes part of other kingdom.