Castle taxation?

Is it possible to change the tax income for castles to be variable like that of cities or is this a hardcoded feature?

Its hardcoded. Castles & Citadels are only "militaristic" settlements.

What do you mean with variable tax income?

Variation with regards to low, normal, high, and very high.

Ah, then it's probably hardcoded indeed, as SilentResident wrote.