Hard Anti-King Sudoku

No two identical digits may touch diagonally (like a chess king's move). Every cell has up to 8 restricted neighbours.

Easy Medium Hard Expert

Hard Anti-King Sudoku. Long elimination chains weave through diagonal and standard constraints — pencil marks and systematic initialization are essential.

[
[
0
,
4
,
6
Mistakes
0/3
Score
-
Time
00:00
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Progress0%
♚ King's move constraint active

What is Anti-King Sudoku?

Difficulty
★★★☆☆
3/5
Constraint Type
Anti-Constraints
Typical Givens
22–28
Avg. Solve (Hard)
26 min

Solving Techniques for Hard Level

Technique Description Level
Diagonal Exclusion The king constraint extends the exclusion zone to all 8 neighbours including diagonals. Apply diagonal exclusions before standard row/col/box logic. Beginner
Corner Restriction Corner cells (e.g., r1c1) only have 3 king-move neighbours. Start with corners and edges to build initial placements. Intermediate
King-Chain Elimination When a digit is placed, eliminate it simultaneously from all 8 surrounding cells. Intermediate
Parity via King's Move In Anti-King, consecutive digits often become mutually king-move incompatible — trace valid placement paths using this property. Advanced

Ready for the ultimate test? Try Expert Anti-King Sudoku — the hardest puzzles on Sudoku.by.

Techniques to Master at Hard

  • Hidden Pairs technique — King-move eliminations quietly remove candidates that would otherwise hide a pair — recount each box after diagonal eliminations and pairs surface where none seemed possible.
  • X-Wing technique — Hard Anti-King grids use diagonal adjacency to break would-be X-Wing cells; verify each corner of your rectangle isn't a king's move from the same digit before eliminating.

Average Solve Time by Difficulty

Easy
5 min
Medium
13 min
Hard
26 min
Expert
48 min
Want a full walkthrough of rules, strategies, and solving steps? How to Play Anti-King Sudoku →

Related Variants

Anti-Knight Sudoku

The companion chess constraint — the forbidden repeat jumps a knight's move instead of one diagonal step.

Anti-Consecutive Sudoku

Forbids a value relationship instead of a position: neighbours may touch, but their digits can't be consecutive.

Non-Consecutive Sudoku

Another global anti-rule solved with the same reflex — every placement immediately bans candidates around it.

Frequently Asked Questions — Hard Anti-King Sudoku

What makes hard Anti-King Sudoku difficult?
Hard Anti-King puzzles have very few given digits and require long elimination chains that interleave standard Sudoku logic with king's-move constraints. A single placement can cause a cascade of diagonal eliminations across the entire board, which must be tracked precisely.
How do I use the king's constraint to break into a hard Anti-King puzzle?
Focus on digits that appear many times in the given cells. Each placed digit produces up to 20 eliminations (row + column + box + 8 diagonal neighbors). Look for a row or column where a digit is restricted to two cells, then check whether the king's constraint forces one of those cells.
What is the hardest type of deduction in hard Anti-King Sudoku?
The toughest deductions involve overlapping diagonal zones. When a digit is nearly placed in a 3×3 box, the king's constraint from adjacent boxes may collectively eliminate all remaining candidates except one. Spotting this requires simultaneous awareness of both box and diagonal constraints.
Should I initialize all king's-move eliminations at the start of a hard puzzle?
Yes. Before making any deductions, go through all given digits and eliminate each one from all eight diagonally adjacent cells. This extended initialization is especially valuable in hard puzzles — it often reveals hidden singles immediately and saves significant time later.
How long does a hard Anti-King Sudoku take?
Expect 30–55 minutes for hard Anti-King puzzles. Experienced solvers who consistently apply the king's constraint can finish in 20–35 minutes.

More questions? See the full Anti-King Sudoku guide.