Gameplay mechanics are not copyrightable

You can only copyright the game, its art, music, etc. Mechanics cannot be copyrighted, or else there'd only be one FPS, one RPG, one TCG... That's why you see so many clones of successful games in the app store for instance. (Remember Flappy Bird?) As long as everything else about your game is distinct, you shouldn't run into trouble. You might get a Cease and Desist at worse, but only if your game goes viral...