During siege battles the combat revolves around control of the castle and its central keep. The deployment zone of the attacker will be outside the walls, the defender's will be around the keep. The object is to reach the keep and stay there and/or rout or destroy the enemy army. Once the attacker gets to the keep a countdown begins, during which they must retain control of the area. Of course, if the attacking forces manage to entirely rout them before that time they automatically win the battle.
When fighting a siege battle as the attacker remember that the goal is to capture the castle's keep, not destroy it! Buildings destroyed on the battlefield will also be destroyed on the campaign map, and must be repaired before they can be used again. When assaulting, try to minimise collateral damage as much as possible, so your prize retains its value. To achieve victory, either kill or rout the entire army, or capture the central keep. Before you begin, ensure that any siege weapons are positioned close enough to target the enemy and their defences.
Defending a Siege
When fighting a siege battle as the defender remember that the enemy will try to either sneak past your defences or rush them by force of arms. Use archers to kill approaching units and melee troops to kill those that get inside the castle compound! The enemy will try to wipe out all defenders, and if your men rout en masse from the castle the battle is over.
Capture of Siege Weapons
Following victory in a battle, and dependent on having an empty space for a unit in your army, there is a chance that the vanquished enemy's siege weapons will be captured by your men. If this occurs they will appear on the post-battle statistics panel. The crew will be replaced by your own men.