That's right, bigger castles can hold more food and soldiers and more soldiers consume food faster. Agricultural production is not so important.
Those sieges will end eventually, maybe 20 years or so which is perhaps an unreasonable time to wait. Unfortunately, I'm not a reasonable person and when I wish not to suffer massive casualties waiting for a dozen soldiers who are hiding in fort knox and I rarely have siege equipment handy. At least a few of my soldiers have probably seen their babies get married while they wait for those bastards to starve. Sadly I am both cheap and patient when I can be.
That is a good reason not to kill all the enemies who rout when they will be forced to hole up in a castle. Remember, you get no extra valour anyways. Actually when I can get away with it financially I prefer to just bribe the army inside and keep the city intact.
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