Thanks! Yeah, I had originally planned on having a UI element (bar or text) to represent the shield charge level but ran out time to implement it.
If the single remaining city was invincible the game would never end and for simplicity's sake this version is designed to be unwinnable - you will always lose, the only thing that changes is how quickly you lose. I definitely think there's something in having the power available be related to how many cities you have remaining but again there wasn't time to implement a mechanic like that for the jam. Yeah, I agree that the projectiles look too much like meteors in this version - I think changing them to be more obviously artificial would make more sense with the re-targeting mechanic.
Yeah, I think I've got some ideas about how to develop the idea further.