The question was asked recently, "What is a castle really? A place of honor? A bastion of good? or the opposite?"
I tend to think of a castle as a haven in a dangerous place. It is a defense for the weak and defenseless. In that regard, the people that run them (be it lieutenant or king) probably set out with noble intentions. They were there, as we may say, to protect and serve. Even if the castle was never used for defense, it gave the people in the area hope. There is much to be said for giving people hope.
From this point, I tend to see a corruption. The people who run the castles come to like the power the castle brings. They come to be demanding of the people they protect. They demand tithes. They demand sacrifices (not in people, but in crops or some such forced gifts). All these demands are for the good of the protectors, which is ultimately good for the people they protect, so the demands are easily justified. Then the corruption goes deeper.
The castle ruler realizes he can get anything he wants from the people “below.” Absolute power corrupts absolutely. The ruler demands things that aren’t just necessary or even a little above-and-beyond, but he demands things to fulfill his every whim. He demands the very livelihood of the people he’s supposed to be protecting. The ruler is all-powerful. The subjects are miserable. Something needs to come along to put the ruler back in his rightful place, whether a besieging army or a silent assassin.
Or maybe that’s just the grim WarHammer GM in me.