You may be familiar with Com2uS from popular titles like Homerun Battle 3D and Slice It. Well, the developer's newest effort comes into a genre that is already more than adequately represented with Tower Defense: Lost Earth. This game is free to download, but as most of Com2uS' games lately have, it relies on in-app purchases to milk you for cash over time.
You're probably familiar with the idea of a tower defense game. If not, it seems unlikely you'd be reading this. In Tower Defense: Lost Earth, enemy creeps will flow into the maze, and your weapon emplacements will have to take them out. You get resources from each kill that can be used to buy more towers, but that's not the main in-game currency. Com2uS uses "diamonds" to unlock more towers and abilities throughout the game. You can get a few in each level by playing well, but to really get moving and have a better experience, you have to buy packs of diamonds.
There are 40 campaign missions across 7 different maps, and 9 towers to unlock and use to blow away the enemy. The graphics have a fun cartoon or even cell-shaded feel. You really can't argue with the price of free, but keep an eye on those in-app purchases. The diamond packs (which start at $1.99 and go up to $49.99) are consumable items, so they won't be restored if you download the app on a different device. Still, you can get by without spending heavily if you don't mind a little grinding. The app is live in the Play Store right this instant.