Unreal Engine-powered tower defense game today, you can have it for free -- developer Trendy Entertainment is now leaning on Android's new in-app billing system to pay for the whole thing. We can't give Trendy all the credit, of course, as Glu Mobile's Gun Bros and Tapulous' Tap Tap Revenge 4 are doing the same thing, but to our knowledge both of them were free to play from day one. Free-to-play gaming has been a controversial proposition in the console and PC gaming space -- most publishers would just like to sell a game once, and call it a day. On phone, however, where apps are expected to be cheap, it could indeed make more sense to charge users for items and upgrades than to have users "buy" the game. Either way, we penny-pinchers are pleased as punch with the idea. PR after the break.