Puzzle Royale

How to play Slitherlink

Draw a single closed loop

Also known as Loop the Loop, Fences.

1. Close a single loop

   You solve it by drawing one continuous loop that closes on itself — no branches, no stray lines, no separate loops — while satisfying every numbered clue.

2. Tap or drag to draw

   Tap the space between two dots to add or erase a line, or drag dot-to-dot to build the loop.

3. Numbers are clues

   A number tells you exactly how many of that cell's four sides are part of the loop. Empty cells can be anything.

Play Slitherlink now

Solving tips

• A 3 in a corner: the two sides hugging the corner are always in the loop.
• Two 3s side by side: the edge between them is a line, and so are both outer edges.
• Two 3s diagonal from each other: the outer corner edges of both are lines.
• A 0 touching a 3 is a gift: the shared side is dead, so the 3’s other three sides are all lines.
• A 1 in a corner: the two sides hugging the corner are never in the loop.
• Lines can’t dead-end: whatever enters a dot must leave. One exit left? Take it. No exits? Back up.
• Never close the loop early — if any clue is still unsatisfied, that little loop is wrong.

Where Slitherlink comes from

• A Nikoli original: it debuted in Puzzle Communication Nikoli #26 in June 1989, created by the editor combining two readers’ submissions.
• Proved NP-complete by Takayuki Yato in 2000 — among the hardest computational problems, despite having basically three rules.
• Known abroad as Fences, Loop the Loop, Takegaki, Dotty Dilemma, and Loopy.
• The finished loop is a Jordan curve — it splits the board into an inside and an outside, and any straight path crossing it alternates between the two. Expert solvers exploit that parity.
• Graph theory in disguise: every dot must have exactly zero or two lines, because the answer is one simple cycle — no branches, no dead ends.

Play it against a hundred other solvers

Puzzle Royale wraps Slitherlink 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 Slitherlink puzzle

The other seven

• 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
• NurikabeShade the wall around numbered islands