Can't get enough of the best Android games which are free to download? Then you may want to check out this 3D tower defense. 

Defenders from game developer Nival is a 3D tower defense and tactic game that puts you into a post-apocalyptic world named Prime World where almost everyone has turned into some kind of monster. Your primary goal is to defend the word's most important resource, Prime from these monsters who want to use it for their greedy intentions. 

What separates Prime World Defenders from most of the tower defense games out there is its focus on the game's storyline. Each mission always has descriptions of why you are heading to that map in the first place. Before and even during games, you will occasionally see pop up dialogues of the characters telling you what to do or what to expect from the upcoming waves of monsters. 

As for upgrades, Defenders adds a unique way to upgrade each tower. After each mission, your player gets a card or several card towers which you can 'forge'. Forging two identical tower cards will allow you to upgrade that particular tower in-game. There are also rare cards which can be 'enchanted' into your existing cards to add specific effects like increased attack speed or higher damage. 

Another way to upgrade in order to make finish missions easier is to upgrade your 'talents'. It includes a number of skills which would passively and permanently increase certain attributes. For instance, a certain talent can reduce the cost of building towers, while another adds one tower slot, allowing you to bring more kinds of towers to battle. 

Prime Defenders is a decent game with a cool gameplay, especially considering its 3D graphics. However, you better be warned that aside from the game's in-app purchases, its size is about 500MB, so you better stay close to your Wi-Fi router.