Aknowledge phase: should a reign challenge another to war; the defending one would have a time to aknowledge the challenge ( more or less than a Day) so the defending kingdom could warn its citizens and prepare for battle, BUT MOST IMPORTANTLY: Set the battle time that will follow during War phase
War phase: the attacker would then have a couple days which they could put their siege in action letting the defending kingdom at guard while still having some kind of surprise element since the defenders would still be unsure when they would lend their blow ( 1 week maybe ? )
* during the war time that would be decided the attackers would then have a bonus damage against the kingdom's city shield so the siege could last a couple of hours BUT....
~ Since the damage bonus to shields would make siege so quickly the attacker could simply wait til favorable time zone so they could do it in less time and without any resistance at all
-> to counter this: both sides should state through ingame menus the best hour for both sides during said "aknowledge time" with the attackers having their way should the defenders failing in check their disponibility
* both side MUST set a battle time during "aknowledge phase otherwise it would be automatically set, consequently favouring one side (this would avoid the defenders from forfeit the challenge)
BUT THEN: the attackers would use the bonus to maximize the damage output while still camping the city and lowering the shield till they eventually win
* some changes should be nade in order to get the best out of this buff:
-> with the battle time ending the attackers should not be capable of hurting the defenders shield til the next round, Also...
-> the defenders should bot be able to repair nor raise their shield for the entire War.
-> the attackers should only be able to make limited maximum damage per round ( 25% would require them to win 4 battles to totally drain the shield in 7 days war the defenders would have to win 4 of them aswell so they could hold their city's defense)
-> the defenders shield would be imune to a reign that is not challenging it for the entire war.
-> there should be a limit to the number of siege weapons that could be built by both sides in a radius ( example: you should have only 4 catapults built at time in a round next to each other) this should prevent the attackers from outbursting through the shields in less than 10 minutes with several siege weapons or so)
* A battle day would count as won for the attackers by making the maximum damage input allowed
* while for the defenders they would have to prevent the enemy max damage output for the entire battle time
* a war is only won by the defenders if the enemy cant bring the remaining of the shield down with the amount of battle days they have lefting
OUTCOMES:
The attackes won: the majority of the battle days and are free to destroy the walls as well as the city or whatever they have come.
The defenders won: having won the majority of the battles their shield is still up and imune to damage for a couple days, repair shield is allowed again aswell as regeneration.
Whatever the outcome may be, the defending side cant be challenged again in war for a time period set by the devs
And thats my idea

Bellow are some extras to add a more time management and stuff to this:
*Balancing the war phase with maximum damage input on shields per day: both sides have RL stuff to do to maximize efficiency in both sides they should be able to adjust the war phase as they see fit, fewer battle days would have battle times with more maximum damage to shield (a 3 days war would allow 50% damage to be made each day during the battle time; making so that the attackers would have to win 2 battles out of 3)
* The sides could chose how many warriors would attack and how many would defend while still keeping their numbers a secret, this could add to the max damage output as man power
The attackers OR making them be able to built more siege equipment (with a stat requirement for preventing them from listing bots for the war just to own more damage or siege equipment allowed)