[Taqwus]

A note on the castles:

Pay attention to the Admin rating; that affects how much it supplies nearby provinces. The more obvious Supply rating governs how much supplies it stockpiles in case the castle itself is sieged.

There are also magical sites which affect supply; mostly, if not entirely, nature. Nature's a great "support" path if you have living troops because of the supply items, regeneration, elemental resistances et al that it offers.

Starvation can take place pretty rapidly. If you just cut off an enemy force from its main supply source and you attack the very next turn, the enemy may already be facing starvation (-4 morale penalty). And keep in mind that mere ownership of a province cuts off supply lines; if a few Call of the Winds sufficed took the province, the line's cut even if the CotW force could be destroyed trivially the next turn.