I never have much luck with defending keeps and forts, the defences just aren't suficient to offset any numerical disadvantage.
For castles and above, I garisson with a single unit of foot knights, a good unit of archers (Longbow, brigands, vanilla if you have to) and gunpowder artillery if you have any, a serpentine or two or a volley gun maybe.
I've held out in sieges like that against massive armies, because casualties get too high for the AI to bother with, unless their really determined.
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