The puzzle sets for this game are sampled from the 3 million Sudoku puzzles with ratings created by David Radcliffe at kaggle.com.

Three millions is colossal, and I decimated it to a little over three thousand sets.

David's original data set includes answers, which is omitted here to make it even lighter for the server to handle. In any case, all that is needed to validate the entries is to check them according to the rules.

What I like about David's data set is that it comes with difficulty rating for each set, ranging from 0 to 8.5. I grouped the difficulties into five different levels, from Easy to Insane.
But when I played a few of them I quickly discovered that on many occasions even the 'easy' ones are not easy. That's why I have calibrated the minimum difficulty level to 50. Generally speaking, the higher the game number, the more difficult the game is. I warn you, you may find the difficult ones really difficult, possibly frustrating and annoying.

Difficult ones are difficult because there are more crossroads where you have to try one number or another.

-- Lucian