The castle/village relationship

The best way of getting a castle or town is to capture one yourself. Keep an eye out along the borders where towns and castles frequently change hands. Those places usually have a rather weak garrison so you should be able to take it without too much trouble.

When you capture a town or castle, you can make the request to your king that you want to keep it. Unless you've just recently been rewarded a fief, the King will almost always grant you the town/castle you've just captured. Don't count on the king giving you one otherwise unless your reputation with him and the other lords are all super high and you manage to convince them to support your cause.

If the castle near your village is already owned by a lord (almost 100% of the time it is), then you are out of luck, you can't get that one unless an enemy faction captures it and you retake it.

The maximum amount of troops on the battlefield is determined by a setting in the options menu, with a maximum of 150 total. the ratio of troops on either side is determined by the total amount in both armies and the relative tactics stat of both.

For instance, with 200 solders, fighting a group of 100, with even tactics scores you would get 100 men on the battlefield and they would get 50. fighting a thousand, you would get 30 or so to their 120. Once the ratio has been changed, (by death) then each side gets reinforcements at gradually slowing intervals until the unused fighters are used up. If an army is entirely defeated before the reinforcements arrive again, another battle is fought, until one of the armies is exhausted.