Puzzle Royale

How to play Nurikabe

Shade the wall around numbered islands

Also known as Islands in the Stream.

1. Wall off the islands

   Solve it by shading a connected wall that separates islands of exactly the right size — one island per number, matching its cell count.

2. Tap to shade the wall

   Tap an empty cell to shade it into the wall, again to drop a dot ("this is island", an aid), once more to clear it. Drag to shade a run. Numbered cells are always island and never shade.

3. No touching, no 2×2 pools

   No two islands touch side to side (corners are fine), the wall must stay one connected shape, and it may never fill a whole 2×2 square.

Play Nurikabe now

Solving tips

• Cells wedged orthogonally between two different numbers are always wall — they can’t belong to either island. A great way to open a puzzle.
• A completed island (its white cells equal its number) is sealed: shade every cell touching it.
• Hunt for "unreachable" cells — any cell no number could grow to must be wall.
• The no-2×2 rule cuts both ways: if three cells of a square are wall, the fourth must be white — often the only clue that a cell is island.
• Keep the wall connected: a shaded cell must never be sealed off from the rest, so the cell that would trap it stays white.

Where Nurikabe comes from

• A Nikoli original — the name is Japanese for "plaster wall", after the yōkai spirit said to block a traveller's path at night.
• Known in English as "Islands in the Stream" or "Cell Structure"; the white numbered regions are islands, the black cells the surrounding sea.
• Proved NP-complete via a reduction from the Hamiltonian-path problem — the connectivity rule is what makes it genuinely hard.
• The four rules interlock: island sizes, no touching, one connected wall, and no 2×2 pool — most deductions come from the last two, not the numbers.
• The no-2×2 "no pool" rule is the signature Nurikabe move: it forces white cells that the island counts alone would never reveal.

Play it against a hundred other solvers

Puzzle Royale wraps Nurikabe and seven other logic puzzles in a battle royale: a hundred solvers enter, four rounds, half the field goes out each time, one champion. Or play it untimed in Practice with no stakes at all.

Try a Nurikabe puzzle

The other seven

• SlitherlinkDraw a single closed loop
• ShikakuSplit the grid into rectangles
• NonogramFill the grid to reveal a picture
• MasyuThread a loop through the pearls
• HashiBridge the islands into one network
• Star BattleOne star in every row, column and region
• AkariLight every square with bulbs