Out of curiosity: suppose your players decided to use one of their armies to destroy an enemy fortification which provides a defense bonus. How would you handle it?
I would say they destroy it and it no longer grants a bonus.
I'm honestly not sure what the question is?
Andrelvis, what you speak of is a siege, which is costly for both sides, but resolves itself in one of two (costly) methods:
•attrition (of either side)
•successful assault (upon the defenders)
Starvation and a lack of manpower and supplies (again, either the defenders starving out or the attackers unable to maintain their supply train) generally were in the deciding factor of pre-gunpowder medieval sieges, and direct assaults themselves are difficult and extremely costly in lives, given the defenses of a fully manned and defended castle.
Easier methods probably include the sorts of things adventurers are known for: tunneling/undermining, infiltration (and opening the gates), and so forth.
From a crunch standpoint, you might start with a siege increasing the upkeep and decreasing the morale of both sides, or alternatively, have it decided by a sort of expanded morale/attrition system: when one side's drops to zilch, they surrender or pack up and go home.
Well, there's already an option to do this in the mass combat section with siege engines. Basically, when you damage an enemy you also reduce the DV bonus from fortifications by 1d4.
I'd then say that when the reduction to DV is equal to the bonus from the fortification, it is effectively destroyed.
From a kingdom building standpoint, a "destroyed" building can be repaired by paying half its cost. So, whoever wins doesn't have to start over from scratch when rebuilding afterwards.
Do you mean "in combat", or out of it? If it's out of it - I really don't see why you would want to waste time rolling things out.
If you mean *in* combat, then I'd need to know more - IE, what's the fortification, how do your players propose to deal with it. If its a simple measure of attacking the fortification, I would look at them like a dog who hears a strange noise, and remind them why fortifications exist in the first place: IE, to be attacked.
From a common sense standpoint, yes (although only in regards to walls - watchtowers for example, are quite understandable targets). However, in the rules for mass combat, they don't function as barriers with hit points of their own, but as bonuses to the enemy army.
Well, given the level of abstraction inherent in the rules, I'd go with Tem's advice and allow an additional tactic the army could learn.
Bunker Buster: You target an enemy's walls and other defensive fortifications. Each time you damage an enemy army, you may make a second Offense check. If this second check is successful, you lower the enemy army's DV for any fortifications by 1d4. This tactic has no effect on an army without any defensive fortifications.