Erfworld 119, page 107

[ishnar]

Multiple cities at a time would explain the loss of FAQ, but not the speed and surprise. Jillian would have known what was happening much sooner.

There is no need to do multiple cities at a time. In Rome: Total War. Once all the faction heirs are killed, any remaining cities go neutral.

I don't have to beat every city, just the cities led by Royalty. Then go mop up the neutral cities.