The developer behind the "Bloons Tower Defense" games, Ninja Kiwi, hasn't made a "Bloons Tower Defense 6," as of June 2015. Players can still enjoy “Bloons Tower Defense 5” on the developer's official website.
The goal of “Bloons Tower Defense 5” is to prevent the Bloons from making it from one end of the map to the other. Players have to survive 65 rounds without losing 150 lives. Each time a Bloon successfully makes it across the map, the players lose one life. If a Bloon is carrying a Bloon inside itself, such as a blue Bloon carrying a red Bloon, the players lose two lives if that Bloon successfully makes it to the end of the map.
To prevent the Bloons from reaching the end of the map, players have to place dart towers along the path to pop them. When they first start the game, they are limited to the towers that they have access to. As they level up, they gain access to more powerful towers. Gamers also unlock new stages and modes as they level up.
The key to winning “Bloons Tower Defense 5” is money management. To buy dart towers, players need money. Popping Bloons and successfully completing a round results in money. It's important that players divide the money between buying new towers and leveling up existing towers.