Playing as a guest is fine, but you should always make sure you have your unique game ID written down somewhere safe (preferably not on your device in case the reason you can not access your game is a broken device!). For instructions on locating your ID please see our FAQ in this section about locating your ID.
It does also mean that if upgrade your device and need to switch, you will need to contact us for a transfer. Please see our FAQ in this section about transferring data.
If you get separated from your game and are on the same device, your game ID should be in the recover menu from the login screen