Let's defense ...

It had been awhile since I played any tower defense games on my iPhone! I still remember the very first tower defense game that I play on iPod Touch G1, TapDefense, which is free and playable with so so graphics!
Here is a free Tower Defense that make use of iPhone 4 retina display (so called HD), Treasure Defense HD. better graphics than TapDefense, but really HD, which I can't tell ;)

Another Tower Defense game that is free for now is, Battle Shock. It is a different kind of tower defense game, which you don't really build any defensive towers but you kill the enemies with your magic bolt (maybe that's why it called "Battle Shock") and also get archers to defense your castle. Sadly the graphics look the same even up to wave 30 :(