If you really can't figure out the purpose of this game. Type the password "SOLVE" at the startscreen to see the solution of the first level. Or hit the back-button in your browser and read the manual.
Levels 1-10 are introductory levels that should learn you a couple of basic techniques.
Levels 11-16 are bigger and somewhat harder. Levels 11,15 and 16 aren't necessarily that hard, but are intended to test the player's patience and endurance.
I like to think I succeeded in making levels 17-20 (reasonably) hard. What do you think?