Bloons Tower Defense is a well-known tower defense game that comes in both free and paid forms. If you download the free version of the game, you can play and enjoy a lot of material for free. On the other hand, it might have ads or let you buy extra features or updates inside the app.

The free version of the game usually lets you play in a number of different game types, levels, and towers. You have to carefully place towers to pop balloons (bloons) before they get to their destination. It's both hard and fun. As you play, you can make in-game money that you can use to buy new towers, upgrades, and skills.

You might want to buy the paid version of Bloons Tower Defense if you want to get access to extra content or features, like special towers or maps that are only available there. Most of the time, the premium version doesn't have any ads and may have more game choices.