Windoku 6×6 Extreme — Set 20
Solver Notes
Windoku — Free Online & Printable
Windoku, also known as Window Sudoku or NRC Sudoku, adds four fixed 3×3 window regions to the standard 9×9 grid. The windows are positioned symmetrically: top-left at rows 2-4, columns 2-4; top-right at rows 2-4, columns 6-8; bottom-left at rows 6-8, columns 2-4; bottom-right at rows 6-8, columns 6-8. Every window must contain digits 1-9 without repetition, in addition to standard row, column, and box constraints. The overlapping nature of windows and standard boxes creates rich constraint chains that experienced solvers find deeply satisfying.
About the 6×6 Grid
The 6×6 grid strikes a balance between accessibility and depth. With 36 cells in 2×3 boxes, puzzles take 3 to 10 minutes depending on difficulty. The 6×6 is large enough to showcase variant rules meaningfully — Killer cages have room to create genuine arithmetic puzzles, Windoku windows overlap in interesting ways — while remaining completable in a single sitting. Many players find the 6×6 their "daily driver" format: challenging enough to be satisfying, short enough to fit into a break. Variant rules interact more visibly at this size than the 4×4.
Extreme Difficulty
Extreme difficulty represents the hardest puzzles solvable through pure logic — no guessing required, but the reasoning chains are exceptionally long. Required techniques include forcing chains (sequences of implications that eliminate candidates by showing they lead to contradictions), multi-step coloring, and in some cases trial-and-error with backtracking when chains become impractical to track mentally. These puzzles are designed for experienced competitors and dedicated enthusiasts. Average time: 60+ minutes. Even experienced solvers may spend 10-15 minutes without making a single cell entry while planning a deduction chain.
How to Get Started
- Choose "New Puzzle" to generate a fresh Windoku puzzle with a unique solution
- Click or tap cells to enter numbers (1–6)
- Use the constraint highlights to track which numbers are still available
- Print the puzzle if you prefer solving on paper