Bloons TD 5

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.

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.

Shortly after the release of Bloons Tower Defense, Lore Sjöberg of Wired described the game as cheerful and addictive, calling it "pop culture at its best". In 2012, Justin Davis of IGN described Bloons Tower Defense as one of the best free tower defense games, despite its "amateurish" artwork. He later noted the first entry's lack of depth, and that the sequel improved this.

IGN editor Daemon Hatfield said that he thought Bloons TD 3 succeeded in standing out in the crowded genre by having sufficient core game mechanics and adding an individual twist. The game's iOS version received mixed reviews with some authors praising the unique towers and good gameplay, though the user interface, controls, and lack of leaderboards were criticized. The DS version received similar reception, with Lucas Thomas of IGN describing it as "a pretty good take on good old Bloons TD." Eurogamer's Kristan Reed gave the PSP version 8/10, describing it as "transfixing", though he said that the music appeared to be designed to "drive you insane".