Just thinking back to MTW, I'd like to see the defender given the following options when a settlement is first attacked (i.e. on the campaign map, an enemy stack moves up and tries to initiate a seige/assault).

1. Give battle.
Your forces within the castle march out to meet the enemy in a regular, open field battle (as opposed to having to sally forth later, which can be quite awkward). If you are defeated, the survivors retreat to the settlement and it becomes beseiged.

2. Prepare for seige.
Your forces remain within the settlement, and it becomes beseiged (as currently happens all the time in RTW).

3. Abandon the settlement.
Your forces flee, and the settlement is surrendered to the enemy.