Bloons Tower Defense
Bloons Tower Defense is another game by ninjakiwi, the designer behind More Bloons (review here); there's also an original Bloons, but as it doesn't have badges, I didn't bother playing it. Anyway, apparently, having designed all this dart-throwing monkey artwork, ninjakiwi decided to make a tower defense game out of it, and the result is Bloons Tower Defense. However, while More Bloons is distinguished by its careful and clever level design, Bloons Tower Defense has none of that -- it's the most generic and bland tower defense game you could imagine, and as a result, it simply fails to be particularly interesting.
Anyway, if you've played any kind of path-based tower defense game, Bloons Tower Defense won't have any surprises for you. You have a few types of towers (but not many; only five) and each has a couple of upgrades. Balloons enter at one end of the path, and you have to destroy them before they reach the other end of the path; popping them gets you money, and you also get bonus money at the end of each level. Unlike your typical tower defense game, the balloons don't have HP; one dart or tack will pop them. However, the balloons do come in different colors. Red balloons simply pop, but when you pop a blue balloon, a red balloon comes out (so, effectively, they have 2 HP); this continues up the line to green and yellow balloons. (The bigger balloons also move faster.) Finally, there are white and black balloons, which are slower-moving and smaller, but also produce two yellow ballons when popped. There are 50 levels, each with an increasing number of balloons.
Bloons Tower Defense simply doesn't have anything that makes it stand out from the horde of other tower defense games that exist. Yes, the dart-throwing monkeys are kind of cute, but they don't make up for the fundamental lack of interesting features of the game. Once you figure out the basics of the strategy (namely, that tack towers are a bad investment), you should be able to breeze through the game without too much difficulty; there's simply too few choices to make it particularly challenging. The graphics are quite simple, although the dart-throwing monkeys are cute; there's no sound other than the sound of balloons popping, which could definitely use some variation, and there's no background music, either.
Overall, there are simply vastly better tower defense games than Bloons Tower Defense. In its defense, it does move the action along pretty quickly, so you won't really feel like it's a dreary slog if you're playing it to get the badge (like me!), but it's not really interesting action, either.