There is a good reason why there is a judge who takes care of the taking gold, items, banning, deporting, executing.
If having these personal prisons addresses and improves on something that is currently lacking, then please explain what those shortcomings are. Otherwise, I think it is an unnecessary feature that complicates the game for everyone who has a unit with benefits that might not justify forcing every single player who takes a unit into battle to be alert about prisoners.
