Stop the hoards of enemies by placing towers in strategic positions. Place more towers or upgrade the ones you already have if it looks like enemies are getting through. Try to spend only as much as you need to defeat all the enemies each round, as you gain interest on whatever money remains at the beginning of each round. If you currently can't afford a certain tower or an upgrade for an existing tower, the corresponding button will be dimmed as an indication. A new wave of enemies will spawn every thirty seconds, as indicated by the timer at the bottom of the menu, and the slider on the left of the menu. Each enemy has different strengths and weaknesses (for example, flying enemies can only take damage from towers that can attack air units).