Mount & Blade Warband How I became a Vassal enjar ( 64 ) in mountandblade.
(I have no source for this, but I did capture a great city alone and the king gave it to me directly). If you conquer a castle yourself, like when conquering a village or fief, you can ask your king to give it to you and if they don't you can either renounce your oath and fiefs, renounce your oath alone and take your fiefs and the one you just captured thus causing you to rebel, or you can take a payout. Now i asked it again, and because he already gave it as a present to another 'friend', he was able to give it to me.I would suggest trying to capture one alone and you should have a higher percentage to get it.
So then it was the second time i conquered the castle alone (This time was easyer because there were only recruits present in the castle, and i gave them free passage if they left). But because i was a nobleman i let it slide, now the fun part was that about a few moves after the castle was again under enemy control. When i finally captured my first castle the king said he would prefer it to give it to his friend instead of me. In my time i had 3 little fiefs, no castle and no large city. The more the king likes you the more chances you have of him giving you the castle you conquererd.