when you are on the offensive side, you are going to lose a lot of men. sieges are mostly archery battle. Okay, this is how I do it. Let's assume you have can 200 men maximum. you can't have that many men, gain some renowns and put some points on leadership.
First I try to spawn 25-30 infantry units and 30-40 archers. My battlesize is about 150. I have to one of my companions max out on tactic so that you can face equal number of enemies even in the cases when you are greatly out numbered. the first group of infantry are crap units with shields. they are not top tier units. you tell them to go forward 10 or 20 steps. their job is to attract enemy archers. the archers i spawn first are pendor armored archers. after they run out of arrow, you tell the archers to charge! the infantry stays.
After your first wave dies out and get first wave of reinforcement, don't let them charge. You hold them and put them behind your infantry. My second wave of archers are Ravenstern Rangers. You tell everyone to charge after they run out of arrows.
Okay now, you have to decide if you can take the castle. Because if you do, you are going to lead the charge. My second wave of reinforncement are top tier infantry and archers. for example, I have halbardiers, kiergard, and knights of ebony gaunlets. While the archers are shooting each other, you go up the stairs. You jump over the enemy line and climb up the ladder to either side. You tell everyone to charge. The enemy units will be busy fighting your dudes at the choke point while you snipe the enemy with arrows.
If you copy my system and understand what why I do what I do, the game is essentially spoiled.
The reason I have different units - such as bringing pendor armored archer, ravenstern rangers, knights of ebony gaunlet rather than just one same unit of archers is because I want my guys to spawn a certain way. I want to have the right ratio of infantry and archers. Shields is very important. If you have guys without shields such as halbardier, you want to put them last and hopefully by the time they spawn enemy archers are all dead. Halbardiers are awesome in sieges both offensively and defensively if you can keep them away from arrows.
Defensively I never had much trouble. Your archers do most of the killing, and most of your infantry should have shields except a few damage leaders. My raio of archer and infantry is 75-25. Most of my companions have either pendor 2 handers or glaive. They chop people down really well. I also have a ruby glaive myself, but it is over kill.