Play King Killer Sudoku Online Free

Play King Killer Sudoku online for free. Killer cages combined with the anti-king constraint — diagonally adjacent cells cannot contain the same digit.

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How to Play King Killer Sudoku

  1. Standard Sudoku rules: every row, column, and 3×3 box contains 1-9.
  2. Dotted cages show a target sum — digits inside must add up to it, with no repeats inside a cage.
  3. Cells a king's diagonal step apart cannot contain the same digit (anti-king).
  4. Each placed digit eliminates itself from up to 4 diagonal neighbours, plus the standard row/column/box uniqueness.
💡 Pro Tip
After every placement, sweep the 4 diagonal neighbours and eliminate the digit. Together with row/column/box rules this means any 2×2 block must contain 4 distinct digits — a very fast deduction tool. For L-shaped cages spanning a 2×2 region, the anti-king rule eliminates entire pair combinations from the cage total.

Frequently Asked Questions

What is the anti-king constraint?

Two cells that are diagonally adjacent (touching at a corner) cannot contain the same digit. It's named after the chess king's diagonal step.

Is King Killer easier than Knight Killer?

Different — anti-king is denser locally (each cell has up to 4 diagonal peers) while anti-knight reaches further across the grid. Both reduce candidate sets dramatically.

Why does the 2×2 tetromino rule work?

Anti-king forbids any two cells touching at a corner from sharing a digit. In a 2×2 block of cells, every cell touches every other cell either orthogonally (forbidden by row/column) or diagonally (forbidden by anti-king). So all 4 must be distinct.

Can I bound cage sums using the 2×2 rule?

Yes — any 2×2 block contains 4 distinct digits, so its sum lies between 1+2+3+4=10 and 6+7+8+9=30. When a cage overlaps a 2×2 region, these bounds prune cage candidates fast.

Try More Puzzle Modes

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