I always get addicted to this kind of games! I was happy to see a good balance in the difficulty! I would work on the graphics and the UI, but the main game mechanic works marvelously! Well done!! :)
I would fix the audio being a little too high-pitched on the intro/start, it was not that pleasant, so I muted until the game started. Then the background music was enjoyable, and I felt like living it in the background while working!