Think about this two realms, named A and B, are about to battle at region C. Realm A does an ambush and conceals 3k CS of their 13k CS total at C, and B has 9k CS. Now the smaller army that thought they would have a chance at winning or just a minor loss, got a huge loss. So even though it can help a realm with less troops it also helps realms with a more troops too so you can't say this a good feature just because it gives less troops the upper hand, it gives either side the chance a bonus. One issue with this is, other than no honor or prestige, there is no reason not to always put troops to ambush because it only helps the side that does it because nothing is bad about doing this in battle. So this seems like an overpowered feature.
How about giving the defenders a choice to either avoid the ambushing force or fight the ambushing force first once they discover the ambush? Give this choice to marshals maybe?
If the marshal of the defenders choose to fight the ambushing force, let his entire army face the ambushing force. If the marshal choose to avoid the ambushing army, let his army face his enemy directly. But the enemy army which set the ambush will have to fight without the army they have assigned for the ambush.
Or we can simply make people be only able to set ambushes in certain region types like mountains and forests.
