If you liked the game, please, tell it to all your friends.
When your friends sign into the game, they have the option of writting a "Referal email". In this field, they must fill your email address (the one you are using in this game).
Every time a friend of you register into the game with your email as referral, you gain extra credits that you can change to coins, turns, ...
To avoid cheats, the reward will be credited into your account when your friend has spent 1000 turns playing the game.