IU like the capital idea. Although i believe it would have to be AI controlled unless in defenc in which case the largest player of that race in the region would have to become defender. Once an attack is inbound. and his own cities taken from the field to prevent him having to loose his own armies and city'ies while defending that one.
Ofcourse his cities would continue to operate to raise income/rescources and he could get a gift from the people if he succesfully defends for the aloted time.
It may even be nessasary to have a few defenders so it can be ran 24 hours.