Click "Play" button in the center and re-click "Play" button in opening window to see difficulty selection screen. The game starts after you choose your favourite difficulty level. Your target is defending your tower from the soldiers as destroying them. Remaining energy level is shown in the right bottom side and 10 energy points were adding on each level. How you kill more enemy, you get much money. You can buy strengther weapons in the game breaks with this money. If you click "Q" button at the right you adjust graphic quality. You can use speaker icon to on/off sound effects.