This is my favorite 20x20 classic level by far, for several reasons. Not only is solving the level just incredibly satisfying, but it is a coherent challenge. It is very easy to figure out what the overall goal of the level is, and all of the opportunities for optimization are placed in front of you. There aren't any sneaky tricks, the visuals are clean, and almost every pushable block has a distinct purpose in the puzzle-solving iterations. It isn't just a good puzzle, it's also just great design.
Insights