That's what happens if you buy too many of them. You'll get around 2000+ viruses, which completely freeze the game up. I'm currently thinking of limiting the amount of viruses or limiting the amount of virus towers you can have. Thanks for pointing that out. =)

Thanks - the AI is strong in some situations and weak in others (there are many ways you can trick it into losing), but I'll try to improve on it.

I guess you could try turning the entire thing into an int[][] and detect if all the rows, columns, and 3x3 boxes are fine. However, if you're planning to preset this with puzzles, then just having a few int[][]s stored as the answer keys, turning the entire thing into an int[][], and comparing the two would work.