[Vynd]

You have some extras of this post out there that you might want to delete.

Forts have an inherent supply value. So long as the units inside one require less food than the value of the fort, I think they can stay there forever without starving.

If both of these enemies are down to just once province, then maintaining the siege on them while working their dominion down to nothing does sound like a viable strategy. Marignon's Inquisitors are very good at this sort of thing, building temples all over your enemy's former territory will certainly help too.

But it should at least be within the realm of possibility to storm their castles and kill them that way. It kind of depends on what they have inside of the castles though. If they have, say, a Sphinx, or some other Pretender that is very tough to kill inside of its own Dominion, then you're going to have trouble.

If they just have a lot of more or less standard troops, then what you need to do is build up armies that can get at them inside of their walls. You have powerful fire mages as Marignon, try hammering them with spells. But if you go in with lots of melee troops they are going to get bottled up trying to get through the gates and be cut down by the defenders.