Intially it was released only for app store (Link). Later due to the huge demand and its popularity, as usual like every app developer does, the game had been developed for the android playstore (Link)
I always try to keep my wallet under when it comes to games especially when using apps these days, because its so tempting. If you are addicted to any game, i am sure you will end up with spending some money. If your settings are not right and if it is in your kids hands, don't be surprised to see your CC statements.
So, to buy gems for free within the app, you need to download and install the app called app nana.
1. Play store
2. App store (App joy is the same as AppNana)
you have to create an account first. Once you are inside the app look into those offers which are listed and install apps which gives good nanas.
Hint: Do not install an app right away once you spot it. Review all the Nana offers and then install the one which offers you the most. Because, for the same app, you may get 500 in one offer, while the other might offer you 2500 or so.Once you install apps and keep building your nanas, you can redeem those nanas for real money or gift cards based on your country. For instance you can use 160,000 nanas to get a play store gift card or app store card, if you are in US (If you are in another country, you can also use nanas for play store cards or app store cards if you know a liitle bit of hack, all you have to do is to create a US account)
Once you get some nanas you will be eligible to enter referral codes to gain some more nanas (2500 nanas for one referral) and also you can give your referral codes to friends and family to gain even more nanas.
Here is my referral code : b1776191
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Just add a comment below if you had entered my referral code and i will do the same for you. Thanks! and Have fun attacking and Happy Farming !