If you want to keep your account safe and be able to transfer your progress to other devices, you must first save your account with Facebook, Google Play, or Apple ID.
Here's how you can do it:
1. On the game's main screen, tap on the gear icon on the top of the screen to open the Settings menu;
2. Tap on "Account";
3. Select how you want to save your account by tapping on the button next to each option (Facebook, Google Play, Apple ID).
If you're playing on Steam, you can also save your account with the Steam provider.
4. You'll be requested to log in. Make sure you log in with the correct account as it's not possible to change this later.
5. That's it! Your progress should be safe now!
- Please keep in mind that the only way of transferring your account between devices using different operating systems (Android to iOS, Mobile to Steam, or vice-versa) is by saving your progress with Facebook.