To protect your game progress from being lost, please bind your account with a Game Center or Facebook account. Click on your avatar→Settings→Account→Account Management, and choose Game Center or Facebook to bind your account.
Please note: A Game Center or Facebook account can only be bound with one game account. Therefore please check whether your Game Center or Facebook account has already been bound with another Z Day game account or not before you bind it to your game account.
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Game Guide for Z Day: Hearts of Heroes