Beautiful and addictive, but IMHO your game has an unbalanced level design.
- Level 2 and Level 3 requires a lot of thinking to resolve them.
- Level 4 and Level 5 are very easy and straightforward
- Level 6, 7, 8 and 9 are a little bit more complicated then level 4 and 5, but not as complex as level 2 and 3 (not even close).
IMHO you should swap the levels:
- Level 4 should be level 2
- Level 5 should be level 3
- Level 6 should be level 4
- Level 7 should be level 5
- Level 8 should be level 6
- Level 9 should be level 7
- Level 2 should be level 8
- Level 3 should be level 9.