part of the reason is that morale lacks impact in Bannerlord, and castles lack strategic and defensive value, so battles are too often concluded in the field before it ever gets the chance to go to a castle fight, because there's little reason to run to one.
battles should only end in the field if the forces are evenly matched and gradually wear each other down; if the battle is quickly becoming lopsided, it should end in a retreat to castle, and a siege.
* army A with 1000 men and army B with 1000 men clash in the field
* army A loses the initial engagement and has 300 casualties in a short period of time, which makes morale effects kick in: the army will rout and retreat, ending the fight early and sending both sides back to the world map.
* From here, the player or AI commander of the smaller 700 man force becomes faster and more mobile, so he has the choice to either send his men back into the battle to continue the fight, or (smarter option) retreat to a castle to continue the fight on better defensive terms. Sending your men in against greatly superior odds with no castle should have a morale penalty (also serving as a reminder to the player of the smart decision), while retreating against superior odds should be a bravery trait loss but no morale loss, so that the player is actively encouraged to retreat to a castle for a defensive siege.
* the most strategically viable place to retreat is a castle, as it can be used to harass an army attempting to occupy your territory and send out raids; and is also, ideally, the most defensible location available. so the player wants to go there, as do their troops.
for this to work the following needs to happen to the game:
* defenders need to get more of an advantage to holding a defense in castles, perhaps by making it slightly harder to starve out castles, harder to reduce walls with trebuchets and gates with rams/troops, etc. It shouldn't be impossible to siege a castle though, don't overbuff them
* morale systems need to work better instead of just taking effect when the battle is already lost: the army should on more occasions retreat en masse before they have all died and morale should be gained/lost for options that intelligently favor the men's survival or endanger it.
* castles need to be able to send out autogenerated harassing patrols that can wear down enemy armies in their territory and raid fiefs, so that attackers are forced to take castles in order to secure territory, and defenders are incentivized for retreating there and simply waiting for the enemy to be forced to attack.