Slitherlink

Overview

Slitherlink presents numbers inside a grid and asks you to draw exactly one closed loop around them. A number states how many edges of its cell belong to the loop. A blank cell imposes no count of its own. The line cannot branch, cross itself or split into multiple smaller loops, so local edge decisions must cooperate with the final global shape. The hosted Loopy engine includes different grid types as well as sizes, although a small square grid is the clearest starting point. You are drawing along edges, not shading the numbered cells. A nearly complete board can still be wrong if two tidy rings have formed independently or if one dangling line end has nowhere left to go.

How to Play

Click an edge to include it in the loop. A secondary click marks an edge that must stay empty. Enable Secondary on a phone when placing those exclusion marks, then switch back to draw lines. Work around the numbered cells until the required edge counts are met and the path closes into one continuous loop. Use Undo to correct an inaccurate edge tap. The Type menu changes the board configuration; Restart removes your moves on the current puzzle, and New generates another puzzle.

Tips & Strategy

A zero excludes every surrounding edge, which often forces neighbouring threes. At every vertex, the loop must use either zero or two incident edges; a single line arriving at a vertex may force its exit. Track these vertex constraints alongside the cell numbers. Avoid closing any small ring before the rest of the required edges have joined it. On irregular grids, count the actual sides of each cell rather than applying square-grid patterns from memory. When you get stuck, look for an excluded edge that leaves a vertex with only one possible continuation.

About this version

Slitherlink uses the locally compiled loopy engine from Simon Tatham’s Portable Puzzle Collection. The original authors retain credit under the MIT license. SonoTap provides the browser adaptation and this guide.