Finally got to the end -- I have to hand it to level 10, which is deceptively good at becoming unsolvable, and level 30, the platonic ideal of a hard puzzle in the minimum viable space required. But my bête noire was level 27, which I'm not entirely convinced I could solve again but was relieved when I eventually stumbled across whatever insight it was supposed to generate after bashing my head against it for 2 days!