You should get as much companions as you can get without this thing that they dislike each other. Here is a site which explains what companion likes or dislikes any of the other ones: http://strategywiki.org/wiki/Mount%26Blade/Companions (Note: This site is actually for normal M&B, but in both games [M&b and M&B: Warband] are the same companions). You should also make sure that all of your companions are level 15 or higher and you've given them good armour before you attack a castle (or for really good players even a town). It's very usefull to have a lot of veteran and elite units in your party (around 80 'till 90 or so). In addition, it's more difficult to take a Nord castle than a Khergit one, because the Nords are really good melee fighters while the Khergits are useless in any siege, because they have to dismount. It would also be good, if you or one of your companions would have a high engineer skill, because it reduces the time that is required to build ladders or a siege tower (while you build a siege tower the enemy's lords can come and attack you). After you've conquered the castle, you should put as many units in the garrison as you can, 'cause a castle or town with a poor garrison is usually a popular target for the enemy's lords and the marshall's campaign (in one of my games I put only a few soldiers into the garrison of Grunwalder castle and a few in game days later the Sarranid ruler besieged it, but the nearest Sarranid castle was Weyah or so).
Cheers
Edit: In Native the best way to defend a castle or town is to put a mix of the following units into the garrison: Swadian Infantry, Vaegir Bowmen, Rhodok Veteran crossbowmen, Nord Warriors, Trained Rhodok Spearmen. (not sure whether that are the right names of the units, because I don't play the English Native version of the game that much)