I remember playing this on my phone a few years back. Pretty simple concept, but this is probably one of the best puzzle games I've ever played.
First thing that came to my mind is GMTK's video about tutorials. This game is a perfect example of a tutorial done right, seemlessly integrated in the gameplay, no text needed.
Controls are pretty much perfect on my opinion. When playing The Witness I was really frustrated with drawing lines, it felt really clunky to me. Here it's very easy, and satisfying.