[Lazy_Perfectionist]

What makes independent castles so easy to take?

If its starvation, I have an idea for that I can go into.
If its not enough defense, ditto.

Without knowing the details of the particular problem, I'll be brief.
Create a 'cauldron' commander to assign to every province and supply food. Perhaps a fortress defense bonus as well? But otherwise, a nonfunctional though highly damage resistant unit.