Mount & Blade: Warband

This is easier then it sounds. You should unlock this naturally over the course of the game as you progress and get better and stronger troops. You do have to take 10 places for your own kingdom however. You can't be fighting for a faction. I personally found the Vageirs (pink) and Khergit (purple) to be total pushovers at castle defense with upgraded mercenary and swadian troops. Don't let the number of enemies in these castles scare you as your men can easily crush them if you keep their morale up and help with the fighting. Do your best to take out the ranged units as they can chew your army up. Try to go for the castles and cities that you can just stick up ladders to raid rather than wait 60-80 in game hours building a seige tower as waiting there so long will lower your troops morale and make them more sluggish fighters.