The Airship Challenge is a new feature that players unlock after level 60.
This Challenge is a week long test of skill that will let you unlock alternate art for your Airship (the floating house above your levels.) To start the Airship Challenge, click on the Airship Challenge icon on the left side of the screen, and select Continue. Now, every time you beat a new level, it will register in the Airship Challenge. Once you beat 20 new levels, you will unlock a new design for your Airship!
The Airship Challenge will only be available for a week at a time, so don't hesitate, get to playing if you want to win!