To play the game you need to shoot down the UFOs by ricocheting your shots of the walls. The game gets harder each level and you only have one life for the whole game, so make sure that the Earths shield is not completely drained or it's game over. There are Shield boosts to pick up which will help you out throughout the game. Be careful when shooting as your shots will be destroyed if you hit any force fields. You do have some special weapons to pick up through the game, these will pass through force fiel