Allowing buildings in main city to be damaged
So one idea to force people to defend themselves is to allow buildings in main cities the ability to lose levels if they cannot fend off the attack.
If a player fails to defend their city, then one building at random would lose a level. The player can always upgrade again however it gives them an incentive not to abandon their kingdom. A player who has already paid the 10 coins to upgrade their resource buildings would not have to pay them again, however they should be able to go down past 16 if attacked enough. This would also serve to help lessen the bored of scouring the map prestige hunting.