You can gain access to Challenges after completing The Nikadian Badlands level. The battlefield island is just north of that level, or once Challenges has been opened, you will find a shortcut to it at the bottom of the screen.

There are three challenge battlefields that can be played. Completing a battlefield will award a number of medals relevant to the difficulty of the map. A battlefield will be replaced by another after 16 hours when it is defeated or the replace option is used.

Successful completion of a battlefield requires only that you win with at least one Star. You get the same number of medals regardless of whether you win the battlefield with one, two or three Stars.

Each challenge has two modifiers : Enemy Power and Mana Bounty.

Enemy Power represents the bonus life and attack that the enemies will have. This modifier can take a value between 110% and 345%.
Mana Bounty represents the amount of Mana you will receive after killing an enemy. This modifier can take a value between 125% and 215%.
Medals are the reward for beating a challenge level. You will receive 5-7 medals depending on the level difficulty (this doesn't consider modifiers) or 10 medals if it's a Boss level.

Once you have won 50 medals they will be converted to a Challenge Reward Card Pack. This card pack contains 6 cards that can be any card in the game.