Build towers along the road, to destroy enemy units. All enemy units have both hitpoints and armor. Use towers with good percing to reduce enemy's armor. Sonic tower is very effective against enemies with huge amount of hitpoints but it's useless against swarm of small enemies. Choose any map you like, and try to survive through 40 waves of enemies. Use your starategic skills, while building towers along the road. It's not the game with 15 different towers, most of which are useless. Now, every tower type is important, enjoy!