The game "Bloons Tower Defense 4" requires you to place different automated turrets on a map with the goal of destroying as many balloons as possible from a predetermined stream. Every balloon that passes to the end of the course intact lowers your overall health.
At the start of the game you must choose from one of the available tracks and an overall difficulty rating. Higher difficulty levels consist of a greater number of balloons moving at an increased pace. After choosing the course and difficulty, you must place your turrets on the map. During the first playthrough you only have access to the basic turret, which is a single monkey throwing darts at a relatively slow pace. Each turret comes with a cost, which means you can only place turrets less than or equal to your total available money.
Once you place all of your turrets, click the Start Round button and watch as the balloons pass through the track. When a balloon enters into a turret's radius, it fires at it. Every balloon that passes through a radius unharmed eventually makes its way to the end of the track and lowers your health. If your health reaches zero before all the balloons pass through the course, you fail that level.