Great game!
Makes excellent use of the theme. Good level design and progression, overall super solid entry that I enjoyed.
A few pieces of (constructive, I hope!) feedback.
* There's a bug where hitting the flag multiple times opens multiple next level windows. This can cause levels to be skipped -- I executed this on level 2, which opened levels 3 and 4, then solved 3, which opened 4a. I closed 4a, played the original 4, and then it skipped me to level 6.
* It's perhaps intended, but the graphics look a little wonky when they're scaled off aspect ratio. I think the sharp edges of pixel art might be a bad fit, and perhaps vector or more stylized graphics would be a better fit.
* I'm playing on a slightly smaller screen, and sometimes I had a lot of trouble changing the size of the window enough to complete things. Not sure how to fix this without breaking the core concept, but perhaps some sort of emergency way to change the scale if at full screen?