Toy Defense 4: Sci-Fi Free is an online mobile game, tower defense strategy game, and the player repel attacks of robots and space crafts as he defend his base, is free to play on Android and iOS devices, from Melesta Games.
Download the game at:
https://play.google.com/store/apps/details?id=com.melesta.toydefense4free
https://itunes.apple.com/us/app/toy-defense-4-sci-fi-free/id784542210?mt=8