Loved this game, very cool power management mechanic. The biggest problem that got in the way of my enjoyment was the lack of conveyance. Particle effects are all good but more visual representation of the power flow and how full the shields are would help a lot.
In terms of actual design, whilst I loved the game's mechanics in general, the ending was a little confusing; as in from a logical standpoint I should be invincible because the one city has all the power, however the projectiles all target the one building left which wasn't too obvious. Perhaps a more effective approach would be to have the projectiles continue to attack each city (I thought they were meteors which made the targeting confusing) but have each city provide power. That way, if you lose all but one of the cities, you won't have the power to provide for it but the projectile mechanic doesn't change unexpectedly. Or maybe just a way to convey that they only target living cities would help. Or I'm just dumb.
Minor gripes aside, really cool idea executed well. Do you plan on doing anything more complex with this idea?