Um no.
This is not how things worked “realistically” and never have. The issues came post fighting with logistical support causing issues if you took lands a ways off from your allies/nation, but very few castles were built as stop gaps to bar you from passing through the countryside. The 2nd Punic War and many of the crusades are famous examples of running amok in enemy territory, not just attacking the border.
Refining how this affects the income and upkeep would be a better approach, but to ensure an unpredictable enemy attack, and give incentive to “connect the dots” eventually.
But mystical force fields that prevent you from driving deep into opposing territory to lead an assault should not be added. The game’s current system causes most of these armies to break-up before they can do anything of merit if marching to far. A change does not need to be made to comply with players OCD of how they would like the map to look
Of course it has never worked like that. But it's much easier to do than to add working and feasible logistics, all the necessary systems involved in it, as well as an AI that can actually function with all of that. Why do an extraordinary amount of work, which you will surely agree this would be, if a simple message saying: "You deem sieging this castle/town at this time to be a bad idea as there are still enemy strongholds in your back which could be used to mount a counterattack at any minute and should as such be taken care of first." suffices ?




