Basically there needs to be defined borders, and passing into enemy territory should result in everything from harrasment by the locals to morale reduction to speed reduction.
I think forces need to be forced to limit the time they can move, basically have a "stamina" for a force, which would force them to stop and camp, especially at night.
First: Each faction has a capital, both hereditary (what they will fight to take back with all their might if it's lost) and/or current (if they lost the old capital, there needs to be a temp) and this capital is used to determine distance of support that both minimizes conflict harm where borders meet (the farther from a capital the less resistance an area has) so as a nation expands, the impact its borders has on the others reduces and the resitance from the enemy regions icnreases.
Castles/Towns can then be set to allow ambushes by militia/garrison forces to automatically happen and force attrition in some form, with the capital and towns providing sources of "motivation/resources". The closer to a castle, the more the attacks/attrition events. And it should be an automatic event that isn't actually simulated because it would be like a group running up, slaughtering the sentries, and running away. The better the troops and leadership/tactics of the invader, the more the attacks could be reduced/interupted, but it should still be something that continues to hamper the force. The better the leadership/tactics skills of the governor would work to counter the leaders skills. This would give a reason to have wartime governors.
Then the farther behind borders the force gets, even yet more and more the events happen forcing the attackers to have to either choose to make very risky raids deep into territory or attempt to attrit the enemy from the outside in.
It would be super simple to set up: Each "town" represents a duchy/head county, each castle a county, each village a barony all with their own borders (there could be an overlay you can turn off or on" and each barony automatically calculates a depth when the kingdom changes from gain/loss. This depth would be what provides the majority of the attrition event calculation in terms of frequency and severity.
In addition, forces should have a "skirmish" zone, so if two forces stay close enough long enough without fighting or as the forces simply reduce their range, their outriders begin interacting. They would have multiple zones representing cavalry, archers, and infantry. This would make it so that different forces of different compositions can have meanings: The larger cavalry zone would allow cav units to have an attrition event that has to be countered by cavalry in order to prevent it from being a major attrit. The archer zone would allow archers to have a play in the attrition events and in order to be countered, infantry with shields would need to be present to reduce the damage.
Finally, close enough for infantry and the skirmishers/throwers become part of the events and FINALLY-finally contact.
this means that an all cavalry force could cause damage on an all infantry force with minimal losses without actually fighting, though the presence of archers and spearmen would reduce their effectiveness somewhat. It would also mean that armies would have a reason to be placed in certain areas on the map.
