The Duchy of Lancaster

If he had left castile to its own devices and just focused on his dutchy lands, he would have been an incredibly powerful overlord. So the duchy of lancaster was run like clockwork. And it was this slow sort of bubbling that i think that he was actually really the most effective ruler. He tried not to have his fingerin in too many pies around this time. However, he was incredibly powerful, and he did have the final say on things - which made him quite unpopular.