[HoneyBadger]

My view on this: perhaps if you manage to take a fortress in 1 turn, it's 100%. Each turn after that that you successfully seige it, the fortress's value increases by 10%, so that a careful seige-master who takes his time gets for his trouble an intact fortress, while one that obliterates the castle by whatever method would quickly conquer the province, but lose the fortress built there. The exception would be the home province of nations, where I feel that, if you're strong enough to take the nation's homeland, then you deserve the award of everything they have, intact. Latent magic/holiness could be blamed for this. I could also see high level spells actually causing besieged fortresses to explode (fire magic) if they're captured, causing lots and lots of damage to the army that captured them, or cause the fortress to become haunted (death obviously, also blood for curses), causing problems, curses, and whatnot to befall the new residences, a more powerful death+blood effect could actually cause the haunted fortress to be a source of problems for surrounding provinces, something along the lines of Ju On/The Grudge. Nature magic could cause a fortress to become infested with vermin, while water could cause the very stones of the fortress to melt away like ice. As far as guile goes, if you could sneak in assassins and take out all the commanders in a fortress, or a spy to bribe all the regular troops, you might (with a bit of luck) get them to open the gates to your nation without a fight. The fortress province in this case should, in my opinion, have very low or negative dominion, as the presence of a fanatical province should greatly reduce or eliminate the possibility of a powerful fortress switching sides in the middle of a war. I really like the idea of spies inside giving siege bonuses, as they could poison wells, cause fires, spread disease, destroy food sources, open gates, lose orders, contact the none-too-devout within the city, and ofcourse quietly assassinate a few very tired guards here and there-just stick an arrow in their back, and they don't even have to worry about the rest of the defenders getting suspicious.