I first came up with the Multiplayer idea years ago but my first attempts were filled with holes. Recently I decided to revisit the idea and I'm pleased with the results.
I'm still working on the sudoku database. At the moment there are only 40 puzzles across the two difficulty levels. I plan to adjust the difficulty rating of the current puzzles, add puzzles and add a "hard" difficulty.
I think I need to detect your opponent leaving as I have observed people not started games an awful lot. It's sad that people start a game and don't want to play :(
I'm open to any other suggestions or ideas to improve the game.