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Anti-King Sudoku is a variant where no two diagonally adjacent cells may contain the same digit. Combined with standard Sudoku rules (which already forbid matching digits in the same row, column, and box), this means a digit cannot appear in any of the eight cells surrounding a given cell.

The name comes from chess: a king can move to any of the eight adjacent squares (horizontally, vertically, or diagonally). The Anti-King constraint says no digit may be repeated within a king's move of any placed digit — it's the opposite of the king's reach being unrestricted.

Each interior cell has eight neighbours (up to eight for corner and edge cells). All eight must hold different digits from the placed cell, on top of the standard uniqueness rules. Cells in the top-left to bottom-right diagonal neighbourhood are constrained in ways that standard Sudoku doesn't cover.

Easy Anti-King puzzles resolve faster than classic Sudoku because the extra diagonal constraint eliminates more candidates. Hard and expert puzzles compensate with fewer givens, and the diagonal interactions create subtle deduction chains that can be difficult to spot.

Focus on corner cells first — they have the fewest anti-king neighbours (only 3 for a true corner). Then look at cells near already-placed digits and check how many of the eight neighbours already have that digit. Dense clustering of given digits often creates immediate anti-king placements.

Easy puzzles take 6–15 minutes. Medium puzzles run 15–30 minutes. Hard puzzles average 30–60 minutes and expert puzzles can take 60–90 minutes. The diagonal constraint makes early progress fast but creates subtle interactions in the middle and end game.