The soft lighting, art style and music combine for a soothing effect that counters the tense gameplay. The lighting especially had such a strong impact on the atmosphere.
It seems the dash arrow targets the dash circle closest to the player, which is great because I cannot aim, but on level 15 there is a small issue where there are two valid circles near each other and the middle ground becomes ambiguous as to which it will target (for the human eye at least). It would often send me to the one I didn’t want. Screenshot included.
I found the extra levels 18 and up were fine, polish-wise. I am fond of level designs like 21 that force you to use every ability in the toolbox.
Regarding other comments discussing the skill floor, I’ll add to the sample size that I had no problems myself, the hardest puzzle took a reasonable amount of trial and error to beat, and I was never frustrated with the game’s design. I rarely play platformers.