Samurai Sudoku Strategy: Solve the Five-Grid Cross (2026)

1. Understand the five-grid cross

Samurai Sudoku (Gattai-5) is not five separate Sudoku puzzles. It is one 21×21 board carrying five 9×9 grids arranged in a cross: a centre grid with four corner grids overlapping it. The centre grid sits at rows 6–14, columns 6–14; the corner grids sit at (0,0), (0,12), (12,0) and (12,12). Each corner overlaps the centre in a 3×3 block, giving 36 cells that belong to two grids at once, and every other playable cell belongs to exactly one.

The single rule that makes this different from ordinary Sudoku is the overlap rule: a shared cell must be a legal digit in both grids it touches. A digit that completes a row in the centre grid can still be illegal if it repeats a digit already placed in the same row, column or box of the corner grid that shares the cell. Treat each shared 3×3 block as a bridge that constrains both sides, not as part of one grid or the other.

2. Start from the given clues, grid by grid

As in ordinary Sudoku, the given clues are the only facts you start with. Work one grid at a time first: scan each 9×9 grid for the digits that are forced by the usual Sudoku logic — a digit that can only go in one cell of a row, column or 3×3 box. Mark them in, and only afterwards look at how a placement in one grid changes the possibilities in an overlapping grid.

Do not try to hold all five grids in your head at once. Resolve the centre grid's forced cells first (it has the most shared edges and therefore the most leverage), then each corner, then return to the centre once the corners have narrowed things. The overlap cells are where a placement in a corner grid feeds back into the centre, so re-scan the centre after every corner pass even if you changed nothing visibly.

3. Use the 36 shared cells as the engine

The 36 shared cells are the reason Samurai Sudoku is solvable at all, and they are where most breakthroughs happen. A shared cell has two sets of constraints, so a digit that is merely possible in the centre grid can be ruled out by the corner grid, or vice versa.

The practical habit: whenever you place a digit in a shared cell, immediately check the perpendicular row, column and box in both grids that touch it. If the placement forces a digit in the corner grid, write it in before you move on; if it makes a digit impossible, the placement was wrong and you have caught it early. Conversely, if a corner grid has a row or column with a single empty cell, fill it and then re-check the centre grid's constraints through that same cell. The shared cells are the thread that connects five puzzles into one; follow it cell by cell rather than solving five puzzles and hoping they agree.

4. Propagate, do not guess

Every shipped puzzle admits exactly one completion, so there is always a forced move somewhere if you look hard enough. Guessing in Samurai Sudoku is especially costly because a wrong digit in a shared cell corrupts two grids at once and the error only surfaces several moves later.

When you feel stuck, you have missed a forced cell, not hit a wall. Re-scan in this order: (1) any row, column or box in any grid with a single empty cell; (2) any digit that appears in eight of nine cells of a row, column or box, leaving one cell; (3) the shared 3×3 overlap blocks, where the double constraint most often produces a single candidate; (4) pairs of cells in a row or column that can only hold the same two digits, eliminating those digits elsewhere. Only after all four passes come up empty is it worth spending a hint.

5. Spend the three hints late

Each puzzle gives you three hints, and each one reveals a single correct cell from the unique solution. Used early, a hint just hands you a placement you could have found by scanning; used late, it can break a deadlock that pure elimination has not cracked in two minutes.

The honest order: scan forced cells across all five grids, propagate through the shared cells, and only when you have genuinely stalled — no cell with a single candidate in any grid — spend one hint, then immediately re-scan all five grids, because the revealed cell (especially if it sits in an overlap zone) usually re-opens the cascade on both sides. Treat the three hints as a budget for the whole puzzle, not as three free moves, and try to finish at least one puzzle per tier with a hint unused. That is the only honest measure of whether the strategy is working.

6. Climb the three tiers

The library ships one puzzle in each of three tiers, ordered by given clues rather than by a hidden trick.

• Easy (175 given clues out of 369 playable cells): the most filled grid, so forced singles appear early and the overlap logic is easy to practice. A puzzle here should take a few minutes.
• Medium (140 given clues): fewer givens, longer chains of elimination, and more reliance on the shared 3×3 blocks to break deadlocks.
• Hard (110 given clues): the sparsest grid; expect to spend real time on candidate elimination and to feel the hint budget shrink.

Move up a tier only when you can finish the current one without using all three hints. The mechanics never change — digits 1–9, no repeats, every grid's rows, columns and boxes complete — so difficulty is purely a matter of how many cells you have to reason about at once.

7. The three mistakes that stall a solve

• Solving five puzzles and hoping they agree. A digit that is forced in the centre grid can still break a corner grid through a shared cell. Always validate a placement against both grids before you commit.
• Ignoring the overlap blocks. The shared 3×3 zones are the most constrained cells on the board and the most common source of forced moves. Skipping them to work the outer corners first wastes the puzzle's best leverage.
• Guessing to unblock. Because every shipped puzzle has exactly one solution, a guess that does not immediately contradict a clue can still be wrong in a way you only discover ten moves later, and in a Samurai puzzle that wrong digit poisons two grids. If you feel the urge to guess, you have missed a forced cell somewhere; re-scan from the shared blocks before you commit.

FAQ

How is this different from the rules on the game page?

The game page explains how to operate the grid — tap a playable cell then a digit 1–9, or use the keyboard; arrow keys move between cells; Backspace clears; Undo steps back one entry; Restart reloads the puzzle; Hint reveals one correct cell; the arrows move between the three puzzles; a conflicting digit is flagged on the grid. This guide assumes you know that and plays the layer above it: reading the five-grid cross, exploiting the shared cells, propagating, and budgeting hints. Read the rules first, then work the strategy.

Can I really solve every puzzle without guessing?

Yes. Every puzzle in the library was verified, before release, to admit exactly one completion. There are no puzzles that require a guess followed by backtracking; the forced-move route through the shared cells is sufficient for all three. If you feel stuck, you have missed a forced cell, not hit a wall.

What does a shared cell actually share?

A shared cell sits in the 3×3 overlap between the centre grid and one corner grid, so it must be a legal digit in both. It counts once on the board but constrains two rows, two columns and two 3×3 boxes at the same time. That double constraint is why a placement there is so powerful and why an error there is so costly.

When should I use Hint?

Only after you have genuinely stalled — no cell with a single candidate in any of the five grids. Spend one hint, then re-scan all five grids immediately, because the revealed cell (especially if it sits in an overlap zone) usually re-opens the propagation chain on both sides. See section 5 for the full budgeting rule.

How do the three tiers differ?

By given clues, not by a new rule. Easy ships 175 given clues out of 369 playable cells; medium 140; hard 110. The mechanics — digits 1–9, no repeats, every grid's rows, columns and boxes complete, shared cells satisfying both grids — are identical in all three. Move up a tier when you can finish the current one without using all three hints.

Where can I play it?

Right on this site's Samurai Sudoku page. The mechanic — five overlapping 9×9 Sudoku grids on a 21×21 board, digits 1–9, no repeats per row, column or 3×3 box, shared cells satisfying both grids — has no single author, so this build is written from scratch by Playnesta with no byte-reused upstream files: the rules engine (js/engine.js: grid geometry, overlap detection, placement validation, completion check and total-playable count), the frozen puzzle library (js/puzzles.js: three offline-generated puzzles, each proven to admit exactly one completion), the shell (js/main.js), the stylesheet, the analytics bridge and the touch shim are all Playnesta-original (MIT, see LICENSE.txt in the package). Playnesta Games is the developer and publisher of this web implementation.