What is a red card and how can I win it?
Every time you win a battle, you will receive a red card as a reward. Cards number can be found in your profile.
More you win battles, the more you get cards and badges.
Note that when you extend a lost battle and you win it you will receive a card. However, cards are obtained one time only, even if you kept winning the same battle multiple times.
Currently, you can only collect cards but not use them.
PS: Cards will be used later to open new features, advertise battles and much more.