You must destroy every alien in the formation to advance to the next wave. You will lose one life if struck by any alien bullets. If you allow one alien to touch the ground, the game will be over regardless of lives. The aliens move across the screen in one direction. When one alien reaches the edge of the screen, they will drop closer to the surface and move in the opposite direction.
The more aliens you destroy, the faster the remaining formation will move. You have four shields spread out over the surface that absorb shots, but erode over time. A saucer will occasionally fly over the formation, which can be shot for bonus points.
Large Invader: 10 points
Medium Invader: 20 points
Small Invader: 30 points
Saucer: 50 ~ 300 points