It is kind of funny how Tower Defense games started to spring up from a Warcraft Mod all those years ago. Starting with Element TD, the genre's popularity spread from the PC game to Flash games and eventually to mobile devices.
But as with all games, you can't just take in what the developers want you to know and base your decision to play on that! There are things like having the right balance in gameplay, to being compact in download size, or even knowing if a game is really offline friendly (not all games that Google Play says are, are.)
But with this guide of what we think are the top 8 free tower defense games on Android, it should help with your decision of what to play a bit more. That said, lets start with number 8's Bloons Series by Ninjakiwi.