Beast Towers Free is an online mobile game, tower defense strategy game, humans are attacking your castle, but you will defend it, summon your beasts and defeat them, is free (the first four levels) on Android and iOS devices, from Baptiste Largaiolli.
Download the game at:
https://play.google.com/store/apps/details?id=com.baptistelargaiolli.beasttowersfree&hl=en
https://itunes.apple.com/app/beast-towers-free/id959542405