But that isn't a siege. That's an archer fight over walls, which was pointless.
Pointless only if you have no bowmen! depending on the number of bowman you have, you can do a lot of damage!
And it's a battle than will affect the region morale and production... and if this army can loot, etc... the region will little by little crumbing. Not exactly a siege but the effects would be more or less similar.
And, or course, the defending army, or the attacking one can always attack (as you said).
Of course if not perfect, but (I think) it can be made without destroying and rebuilding all the battle system.
