Bloons Tower Defense (also known as Bloons TD or BTD) is a series of tower defense games under the Bloons series created and produced by Ninja Kiwi. The game was initially developed as a browser game, built upon the Adobe Flash platform and released in mid 2007. Later games in the series expanded to support various mobile platforms, including Android, iOS, Windows Phone, PlayStation Portable, Nintendo DSi, Windows, Linux and MacOS. Games in the Bloons series older than Bloons TD 6 are available through the Ninja Kiwi Archive on Steam. In the game, players attempt to prevent "Bloons" (the in-game name for balloons) from reaching the end of a set course by placing towers or road items along it that can pop the bloons in a variety of ways, typically by using the power of monkeys, magic, or technology. A handful of towers can stall the bloons and give the other towers more time to pop them by freezing and gluing the bloons. Money is gained by popping bloons, completing rounds, and collecting bananas from existing banana farms which can be spent on new towers, upgrades for existing ones, or temporary items such as exploding pineapples and road spikes. They also need to make sure that they do not lose all of their lives.

The main objective of Bloons TD is to prevent Bloons (in-game name for balloons) from reaching the end of a defined track on a map that consists of one or more entrances and exits for the bloons. The game is a tower defense game and thus the player can choose various types of towers and traps to place around the track in order to defend against the bloons, gaining 1 in-game dollar for every layer of bloon popped. If a balloon reaches the end of a path, the player loses lives (or in later games, health); once these are all depleted, the game ends. The bloons always follow the map's set path on the track until they either reach the exit(s), are popped, or are moved to an earlier part of the track by a tower's ability.

There are three classes of bloons in the game: regular (unnamed in the game), MOAB-class and Boss-class. As of Bloons TD 6, the regular bloons consist of: red, blue, green, yellow, pink, black, white, lead, zebra, rainbow, purple, and ceramic bloons. MOAB-class bloons are in the shape of a blimp and consist of: the MOAB (Massive Ornary Air Blimp), the BFB (Brutal Flying Behemoth), the DDT (Dark Dirigible Titan), the ZOMG (Zeppelin Of Mighty Gargantuaness), and the BAD (Big Airship of Doom). Tougher variants of most bloon types contain a number of specified weaker ones. In later versions of the game, regular bloons possess sometimes special characteristics such as camo (which most towers cannot detect), regrowth (the ability for the bloons to slowly grow back to their original size), and fortified (which doubles the health of certain bloons), that resist certain tower types. With each level, the intensity of bloon waves proportionately increases. There are also boss levels, with tough bosses such as Blastapopulous, Bloonarius, Dreadbloon, Lych, Phayze, and Vortex. These bosses usually appear after a 39 rounds. Bosses have very high health in normal boss mode, and have unique abilities. Some bosses have their own strengths and specials. Some installments have challenges, where you have special things.

Towers

Towers, most of which are monkeys, are the main defensive utility in the Bloons TD series. Each tower has its own unique purpose, power, and use, with some being powerful against specific bloons but unable to target others effectively. Every tower can be upgraded to increase power and other capabilities by spending the in-game currency, known simply as 'money', which is earned by popping bloons and at the end of each round. In Bloons TD 4 onwards, certain towers such as banana farms can be placed to produce additional money during a round (end of the round in Bloons TD 4).

In later games, there are multiple difficulty levels; for instance, in BTD5, there are four difficulty levels and in Bloons Monkey City there are six. The higher the difficulty the player plays on, the fewer lives they have and the more each tower and upgrade costs. There are also different types of maps rated by difficulty; in general, there are more types of maps in newer games.

There are currently six released numbered games in the Bloons TD series apart from various add-on packs and spin-offs such as Bloons TD 4 Expansion and Bloons Monkey City. The newest release, Bloons TD 6, was released on June 14, 2018. The series was retitled from Bloons Tower Defense to Bloons TD in 2009 due to infringement on the Tower Defense: Lost Earth trademark owned by Com2uS.

There are currently six mainline games, and many spin-off games based primarily on the Bloons Tower Defense series.