Sudoku ArtA real logic challenge
Sudoku Art's hard level leaves far fewer digits visible at the start. You'll need to chain several deductions before you can fill the very first cell with certainty.
More advanced techniques
On a hard grid, basic techniques (simple elimination by row or column) aren't always enough anymore. Spotting candidate pairs or triples within the same zone becomes genuinely useful to move forward.
Push your limits
If hard starts feeling too comfortable, expert level pushes the exercise even further. Conversely, an easy grid is always there for a more relaxed game.