Challenge: For a quick game and for more fun in standard play.
First you must collect needed cubes for the challenge and then you need to execute the challenge to collect the points. Challenge level is saved! For the next game you can choose to play from challenge level 0 or to continue from highest achieved challenge level.
For a quick game you can simply finish few challenges, advance your challenge level and call it a day.
And for high score games start from challenge level 0 to get maximum of points.