A quick warning, however: Radiant Defense has in-app purchases. Though free to play and entirely possible to finish without the help of micro-transactions, Radiant Defense does have towers that are accessible only if you're willing to fork out the requisite (albeit very small) amount of cash. Still, this really shouldn't deter you from checking out the game.
Sentinel 3: Homeworld
Aliens? Check. Giant mechs? Check. Drones? Missile strikes? RPG-like elements? Check, check, and--wait a second. While Sentinel 3: Homeworld is largely building on the things that made the first two installments work, it also comes with a nifty new addition in the form of a commander unit. The commander unit is a robotic juggernaut capable of dealing massive amounts of damage, healing units, and a whole bunch of other stuff. As you progress, you can level it up and outfit your commander with new abilities--just like in an actual RPG. Aside from that, however, Sentinel 3: Homeworld doesn't deviate much from the standard tower defense formula. Still, that doesn't stop it from being a good-looking, well-made game.
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