What do the difficulty stars mean?

Printed beside each maze (filled and empty stars, five in all), they show how hard that maze measured, from one to five. How to play explains them to the reader. A level's title page shows the stars its mazes usually get: 1 for Very easy, 2 for Easy, 3 for Medium, 4 for Hard, 5 for Expert and Extreme. Turn them off with "Difficulty stars" in "Pages and solutions".