Defend your cave from the invading giants, initially by throwing huge rocks at them! As you progress though, you'll be able to use different weapons to kill the giants more effectively, so hang in there!
What a game! If you'd like more then maybe head over to the HAG Defence section to discover more. To play other games similar to this then try Operation Machine Gun, Defense 2135, Breach Of Death: The Bridge or perhaps just the prequel to this awesome game, Age of Giant.