Mastering Large Grids: Strategy for 10×10 and Beyond

Don't Panic

Opening a 15x15 or 20x20 Slitherlink grid can be intimidating. The sheer number of squares makes it hard to know where to look. But large grids are just collections of small patterns stitched together. Here is a roadmap for tackling the giants.

Phase 1: The Perimeter Scan

Always start with the edges and corners. As discussed in our "Corner and Edge Patterns" guide, these are the most constrained areas. Go around the entire border and fill in every forced line and X.

Phase 2: Low Hanging Fruit (0s and 3s)

Scan the interior for "0"s. X them out. Look for adjacent "3"s or diagonal "3"s. These pairs are independent of the rest of the grid and can be solved locally. Do this quickly to break up the empty space.

Phase 3: Cluster Solving

Don't try to solve the whole loop at once. Pick a cluster of numbers where you have some information (lines or Xs) and work strictly on that area.

Here's the divide-and-conquer approach visualized on a larger grid. Focus on one region at a time:

  ┌─────────────┬─────────────┐
  │ Region A    │ Region B    │
  │ ·═══·   ·  │ ·   ·   ·  │
  │ ║ 3   2  │ │   1   2    │
  │ ·   ·   ·  │ ·   ·   ·  │
  ├─────────────┼─────────────┤
  │ Region C    │ Region D    │
  │ ·   ·   ·  │ ·   ·   ·  │
  │   0 × 3  │ │   2   1    │
  │ · × · × ·  │ ·   ·   ·  │
  └─────────────┴─────────────┘
  Solve A first (has corner 3),
  then C (has 0-3 pair), then
  let lines flow into B and D.

Work Outward: Let your solution grow organically from the solved areas. A line entering a new region restricts that region. Follow the "flow" of the line.

Phase 4: The "Island Check"

On large grids, it's easy to accidentally create two separate loops. Periodically zoom out and check:

  ✗ Two islands (invalid)     ✓ Connected (valid)

  ·═══·═══·   ·   ·          ·═══·═══·═══·   ·
  ║ 3   2 ║       ║          ║ 3   2       ║
  ·═══·═══·   ·   ·          ·═══·   ·═══·   ·
                  2 ║                 ║ 2 ║
  ·   ·   ·═══·═══·          ·   ·   ·═══·═══·
  Loop A    Loop B            One connected loop ✓

• Are my lines forming a closed circle that ignores the rest of the board? If so, that's illegal (unless the board is done). Break it.
• Is there a region of the board that is completely cut off by Xs? If the loop can't get there, that's a problem.

Phase 5: Getting Unstuck

When you hit a wall (and you will):

• Switch Areas: If the top-left is stuck, go look at the bottom-right. A deduction there might send a line snake-ing all the way back to help you.
• Look for "1 or 2" Configurations: Find a cell where only two possibilities exist. Visualize one. Does it immediately crash? If yes, pick the other.
• Aggressive X-ing: Re-scan for "almost complete" vertices (3 Xs means 4th is X). These are easy to miss on big boards.

It's a Marathon

Large grids require patience. Solving a 20x20 is a journey. Enjoy the process of chipping away at the chaos until order emerges. There is no feeling quite like the "click" of the final segment falling into place.