If you're enjoying BTD5, I would suggest Bloons Monkey City and Bloons Tower Defense Battles. City is where you have a city (surprise surprise) and you have to expand out and capture tiles; all of the tiles on the map have a different difficulty you need to overcome, and there are buildings you need to construct to allow you to use towers. Battles is where you are matched up against people online and are allowed four towers maximum to battle them with. It's all about strategy and it's quite a fun game.