Garrison some troops in it and the enemy will generally leave it alone for easier targets. Once you have it make sure you don't lose it, obviously. 95% of the time the king will award it to you. Take a castle or town yourself and then choose to ask for it.
The simplest way to do this would be to get 150 renown and ask a king for a fief. The first thing you want to do is get a castle or town. This is kind of like marrying someone, but the main difference is that instead of getting her relative's approval, you say fuck them and get married anyway. It appears this can only be acquired as a male.
