I love the game and the AI is pretty good too. It's very addicting!
Some suggestions for improvements:
*) Remember the main menu settings. When going back to main menu, it forgets all settings like board size, AI level, etc... Please remember those, it's very annoying like this
*) Not sure if bug, but when the game is over, either win or lose, it just stops, no "game over" or "congratulations" message or anything? I see some one pixel thick red thing move on the right side though, so maybe something is supposed to be rendered there but it's a bug?
*) I didn't hear any background music? Always nice to have some music to associate to the game. Of course with the ability to turn off once heard enough of it :)