Such a simple premise, but well executed and very fun! The difficulty ramps up slowly enough that you have a feel for the game before it gets out of hand, and the art style adds to both the aesthetic and the challenge.
The only potential negative I might note is that the later levels may slightly overstay their welcome, but that's a difficult balance between being long enough that players can demonstrate their accomplishment and being short enough that they will stick it out to the end.