No clue on Droid. But on iOS, you double-click the home button, and then hold down on the app icon. You should see a red circle with a minus sign. Click that, and then press the home button. Then press the home button again.
If you need anything, send me a PM. I'll be glad to help.
Only playing Castle Story these days, but feel free to add me! If you gift me, I'll always gift you back ASAP!