Slitherlink

Hard slitherlink runs on a 10×10 grid with 45% clue density. More cells are blank, which means fewer starting points and longer deduction chains.

Loop closure reasoning is where hard puzzles separate from medium. You need to track which vertices are connected by the partial loop and prevent premature sub-loops. If drawing a line between two vertices that are already connected would close a loop that doesn't include all existing lines, that edge is excluded.

Use the excluded-edge markers (× marks) aggressively. Knowing what isn't part of the loop narrows possibilities just as much as knowing what is. On 10×10, you often solve more edges through exclusion than through direct line placement.