In real life strategy, one of the most important things is to watch your flanks and behind to avoid annihilation.
I imagine a mechanic in which the number of direction from which units are attacking matters. If you attack from more than one direction, the defending side will undergo penalty as the commander of the garnison would need to face attack coming from various sides lessening his effectiveness.
It could be a bonus in damage for the attacker, a malus in moral or defense for the defensor. I have no idea how to balance this idea.
If a defender is attacked by all the possible directions, it is surrounded, applying an even bigger penalty.
One of the interest of this mechanic would be to reward players who coordinate and orchestrate with more tactic their strategies. It will also give a true interest in surrounding an ennemy as i found for the moment surrounding is not that usefull because you need to garrizon all provinces you take to avoid a backstab and it forces you to split your forces, not giving the devisive victory it would have in real life.
Sorry for my english, I'm French,hope you understand what I say,
What do you think of it?