You are the sole protector of an important valley on an important planet. Aliens have decided to invade and send their tanks down the path in your valley and destroy everything! Start with no forces at all and quickly build up an entire defense system to defeat the roving enemies. Place your defenses quickly and as you defeat the enemy used the gained money to upgrade to handle the increase of enemy numbers. Build stronger towers, or just upgrade the weaker ones. Either way strategize as best as you can to save your valley and the entire planet from utter destruction.