A potentially more flexible option would to add a Border Control section to the command menu that provides the option of enforcing different border controls upon different border types. All border controls would default to none but persist from one lord to the next.
It could be left to each lord to set the controls for all border types his region poses. In this case not all regions would have all border types. This would mean that dukes and rulers would need to instruct individual lords on how they want duchy and realm borders controlled. It could also result in inconsistent duchy or realm borders as not all lords HAVE to enforce the same controls along their stretch of duchy or realm border. The local lords also get stuck with the bill but they could potentially negotiate funds from the requester to "cover the expense".
Border controls could also be set by position. Region lords can determine the border controls for their region and fork the bill for that. Dukes determine what controls are enforced on duchy borders and pay for those controls and rulers manage and pay for realm border controls. When a region, duchy and/or realm border overlap the strictest controls apply.
So the border types might be:
Region Border Control
Duchy Border Control
Realm Border Control
Some possible options as to what controls can be placed on a border (could vary by border type):
Limit/prevent peasant Migration
Report/impede (all, non-allied, enemy) troop movement
Report/limit/prevent Report (all, non-allied, enemy) noble movement
Capture (all, non-allied, enemy) infiltrators and/or scouts
This could all be mixed and matched too. An example region might have the following border controls:
Region Border controls: Limit peasant migration to 5%, capture all enemy infiltrators, impede enemy troop passage.
Duchy Border Controls: Limit peasant migration to 2%
Realm Border Controls: Prevent peasant migration, capture all enemy scouts, report non-allied troop passage, capture enemy nobles, report non-allied nobles.
They all costs a bit of gold to start (for building the needed infrastructure, training, hiring), have maintenance costs and could involve a morale penalty. The start up and maintenance costs depend on how much border needs to be patrolled/controlled and what controls are in place. Morale penalty depends on how limiting the restrictions are for the residents of the region/duchy/realm. Slightly limiting migration across the realm border would cause slight morale penalties and completely baring all migration out of the region would cause more severe penalties to morale. Some controls might even boost region stats due to the sense of security they may provide.
A bit convoluted but I think it makes sense after the second read...
