Bloons Tower Defense

Bloons Tower Defense (also known as Bloons TD or BTD) are a series of tower-defense games that were created by Ninja Kiwi. The focus of the games is to use towers to defeat an enemy called Bloons to progress through the game. As you play the game, the Bloons become more difficult to defeat. Players are able to upgrade their towers to help defeat stronger enemies. Bloons can come with power ups, such as camouflage and regeneration. Most of the Tower Defense games can be played on computers and mobile/tablets.
Pricing.
On mobile, all of the Bloons Tower Defense Games (except Bloons Monkey City and Bloons TD 3, which are free on the App Store) cost $2.99 to play and download. A free game is also on their website. Some of the Bloons games have lite versions, such as Bloons TD or Bloons TD 4, which are free to play, but have limitations.
Bloons.
The enemies of the series are "Bloons". (The trademarked word Bloons is a spinoff of the word "balloons".) Once the Bloons come, a monkey will have to be placed to pop them. These monkeys pop the bloons. Red Bloons release no "children". B.A.Ds also have no "parents". Children &amp; parents are terms determining what Bloons release when pop (children) and what Bloon releases that Bloon (parents). For example, the child of a Blue Bloon is a Red Bloon, and the parent of a Blue Bloon is a Green Bloon. A Red Bloon Equivalent (RBE) value is the number of Red Bloons it is equivalent to.
Towers.
In Bloons, there are a lot of towers. All towers contain upgrades that help make them better. All towers can be upgraded four tiers. You can only upgrade one side of the tower to the fourth tier but upgrade the other side to the second tier.
Popularity.
The Bloons Tower Defense series has been a top seller on the Apple app store. Bloons TD 4 reached No. 6 on Apple's list of top paid iPhone and iPad games in 2010. Bloons TD 5 was able to reach No. 8 on Apple's list of top paid iPhone app downloads in 2013.