A great idea, really elegant in its simplicity. Reminds of me Suika Game in how it starts of simple but gets very tricky the further you get.
With a game like this, I'd like to have a point goal built in so that I can feel like I know when I've "Beaten" it.
Another idea if you were to take the game further would be to build in a reward for placing a piece in a more difficult way could add some more depth without affecting the complexity.
Overall, I really liked it, great job!