Since I started playing Addictive Tower Defense I have been going out of my way to find other tower defense style games. The latest to join my arsenal is Kill All Bugs! by Sonic Boom. The game is a take on traditional tower defense games where you are protecting humanity from mutant bugs (Men in Black style!).
To play the game you simply need to build up your arsenal with heavy weapons to defend your territory. If you have ever played a tower defense game in the past you should pick it up naturally. Its actually a pretty fun game which you can purchase for $4.99 over @Bplay.