The main gameplay is to play as Demon to stop Runner's attack and collect five gems to win. Or play as Runner and successfully invade Demon's castle.
There are different decks for different camps. I would recommend learning the tutorial first, otherwise you will struggle to understand the rules of this game. But the pace of battle is slow, and other decks cost 1500 crystals to unlock.
I'm not very good at this game and am losing a lot to my opponents at the moment.