Playing on multiple Android devices is easy! First thing you need to do is log into Google Games on your device that has the Fortress you wish to use on multiple devices. You can follow the instructions in the following support article to do this:
Next, on your second device log in and complete the tutorial. Don't worry too much about your town name since you will overwrite it.
Follow the same instructions you used to connect to Google Games on your first device. After you log in you will be asked to save your town:
Make sure you tap the green checkmark to overwrite your current Fortress with your saved one. This will allow you to play the same Fortress on multiple devices!