Its a fun game but the AI is not smart enough, I think with these kind of games multiplayer is what make it challenging. Great work dev add multiplayer and this game would easily be the game of the month.
A suggestion if you want to make it even harder consider a 4x4 or even 5x5 square with the aim of connecting 3 or 4 in a row would definitely add whole layer of complexity and funm =).
Where could we put them?