Play Fortress Sudoku Online Free

Play Fortress Sudoku online for free. Shaded fortress cells must contain a digit greater than all orthogonally adjacent non-fortress cells.

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How to Play Fortress Sudoku

  1. Standard Sudoku rules apply.
  2. Shaded 'fortress' cells must be strictly greater than all adjacent non-fortress cells.
  3. Unshaded cells adjacent to a fortress cell must be strictly less than the fortress.
💡 Pro Tip
Corner fortress cells with 2 non-fortress neighbors must be at least 3. Edge fortress cells with 3 neighbors must be at least 4.

Frequently Asked Questions

What does 'strictly greater' mean for a fortress cell?

The fortress digit must be larger than every orthogonally adjacent non-fortress neighbor — equal values are not allowed.

Can two fortress cells be adjacent?

Yes — the constraint only compares fortress cells with their non-fortress neighbors. Two touching fortress cells don't constrain each other.

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