The Rules:

The aim is to protect the four cities from falling asteroids. To do this you must shoot them as they fall, either by clicking on the screen or by using a detonator (hitting the space bar) to destroy them all. You receive ten points for every surviving city at the end of each level. Cities are replaced every fifth level, but if all your cities are ever destroyed, the game is over!.

Ammo:

There are fifty levels. You start off with five shots per level up to level five, then six shots up to level ten, after which you have seven shots per level. You only have two detonators to last you the whole game so use them wisely!

Good luck!