Have you guys played Tower Wars yet? It's a fun tower defense game, but it's a multiplayer game mainly. The point is basically to send your own troops against the enemy or opponent, and try to destroy their castle. But you have to get through your opponents defenses to destroy the castle. If you play the game right, you can upgrade your units, get even more powerful units, and get your way through your opponents defenses. Thing thing is your opponent is also sending some of his units at your castle, so you have to build an ultimate tower defense against the enemy as well. All in all, I have well over 150 hours on this game, and it is still fun to play.
Don't they have this for mobile? I think I came across it in the Play Store.
Not that I know of, but they had a good deal this week, you can get it for 3 people at once for 12 bucks I believe. Something like that. It was on steam. Or one for 4-5 bucks. You should see how most of us have this. Shannon has it, I have it, Aditya has it, and a few other people too. It's a fun game. We like to play it when we want to relax for a bit.
Though if you want a fun tower defense for mobile, you should check out Fieldrunners. As far as I know it's available for iphone and ipod touch, though not sure about android. I'm sure they have it for that as well. Very fun game, played it a few times on boring lectures in class. :P