What a great game! Trying to fill all these square lines with round tools is a fun challenge, and somehow it really does feel like building a sand castle.
One thing I’d add, though this is not really in the spirit of the game so far, is a Speed Mode. Imagining it would show the score in real time and you can stop when satisfied. Would give it a meatier challenge for those who want more, like “how fast can I reach 70% in all the castles?”… I might play that all day :D