One more thing, if you are thinking about taking a castle you should try to capture as many refugee country troops as possible and recruit them into your army. Just keep them as prisoners in a regiment and stash that regiment some place where it won't be attacked by anything. When you get a couple hundred start recruiting them. This way you won't have to pay for them until you need them and once you have them they won't cost you a fortune to keep throughout the game.
Also, forest bandits make pretty good castle assaulters because their arrows kill pretty well. And even though they are more likely to die at the beginning of a battle (I put them at the top of the list so they spawn first) they will likely drop arrows for you to shoot at the defenders. This is a huge advantage and they only cost you 5 denars a week to support.
Finally, you only need to put a few men in each castle. You can take all the rest and keep taking castles until you run out of men or castles to take. Why do this? For starters castles are strong points that can hold off attackers for a while. When they are under seige you just bring in a larger force from somewhere else and run the attackers off. So long as your don't go too far away from your castles and have a mobile force to pour into the castle to help defend it you can have a lot of castles but just a few men.
There is an old Holy Roman Empire (Germany) saying that expresses this strategy, "He who defends everything defends nothing."
Right now I've got 3 castles and one city (Yalen) and I don't have more than 300 men TOTAL. I am about to conolidate what I've got and attack Jelkala since it's been weakened by the Kergits and Swadians.
I will put a permanent garrison of 10 men in Jelkala and use the rest as a reactionary force to defend my realm. I may even take Veluka as well.
Someone asked how you take cities with just a few hundred men? First you need lots of archers. Probably two thirds of your army or more depending on if they have good combat skills. Put them all at the top of the que and let them kill the defenders on the wall. Keep any infantry that spawn up close to the wall where they won't be under fire. Sometimes if you put them just to the left of the ladder the defenders turn towards them and expose their unshielded side to your archers.
When all the archers have run out of arrows and you have fired all of yours, then send them in to attack the castle. If you have high surgery most will just be knocked unconscious. When your reinforcements arrive recall all your archers and let the new archers loose all their arrows as well. I don't remember how many reinforcements you get but I think it is three. When you run out of archers send in the infantry. Their weakest troops should spawn last and it will be an easy battle.
Remember, that it doesn't matter if the have 10,000 men in that city. They are only going to be able to bring a few hundred to bare against your army. Then there will be a second and third battle. If you can win those then you capture all the remaining troops! So make sure you deploy a regiment before you assault the castle so you can soak up all those prisoners. To do this, just wait until the ladder or tower is built and then when the game says you are ready to attack go to Party and talk to one of your troops and tell them to become a party. Then briefly continue the siege and then click on the game to bring up the attack options again. You will notice that the regiment has joined the battle. Now you KNOW you will capture troops. If you do it too soon then your regiment may be run off by enemies or bandits or it may engage in battle and wont' be able to assist with the castle battle.
Best troops for attacking castles are Villianese religious troops. They come with great armor, weapons, and powerful bows. Put them at the top of your cue and you won't even need infantry.