Easy: naked singles. Medium: hidden singles. Hard: locked candidates or pairs.
Expert: triples or X-Wing. The hardest technique actually used determines the level;
score totals the weighted steps. Ratings depend on this solver's technique order.
Verification
The server generates a complete valid Sudoku solution,
removes clues, and tests every removal.
A clue is only removed if the resulting puzzle has
exactly one solution.
When you press Check, the current grid is independently
tested for validity, solvability and solution count.