I built Slitherlink Online because every existing Slitherlink site had the same problems: ugly UI, no difficulty control, no way to track progress. Here's what's different.
What is Slitherlink?
Slitherlink (also called Fences or Loop the Loop) is a logic puzzle invented by the Japanese publisher Nikoli. You're given a grid of dots with numbers in some cells. Your job: draw a single closed loop using the edges between dots, where each number tells you exactly how many of its surrounding edges are part of the loop.
It's pure logic — no math, no vocabulary, no cultural knowledge. Just pattern recognition and deduction. That's why it works across languages.
The Puzzle Collection
3000+ hand-quality puzzles generated by a custom Rust engine (not random placement):
| Grid Size | Cells | Difficulty Range | Typical Solve Time |
|---|---|---|---|
| 5×5 | 25 | Level 1-4 | 1-5 minutes |
| 7×7 | 49 | Level 2-6 | 3-10 minutes |
| 8×8 | 64 | Level 3-7 | 5-15 minutes |
| 10×10 | 100 | Level 4-8 | 10-30 minutes |
| 12×12 | 144 | Level 5-8 | 20-60 minutes |
Every puzzle has been verified to have exactly one solution. The difficulty rating measures what solving techniques are required, not just "how many clues are missing." A Level 1 puzzle needs only corner rules. A Level 8 puzzle needs loop closure logic and multi-step reasoning.
Daily Challenges
Every day at midnight UTC, a new puzzle is selected as the daily challenge. Everyone gets the same puzzle. Solve it, submit your time, and see where you rank on the leaderboard.
This creates a shared experience — you can discuss today's puzzle with friends, compare strategies, and see if your approach was efficient. The daily challenge rotates through different grid sizes and difficulties.
What the UI Actually Looks Like
- Dark mode only — no eye strain during evening puzzle sessions
- Emerald accent colors — clean, minimal, professional
- Touch-optimized mobile UI — tap edges to draw lines, tap again for × marks
- No ads (yet) — the focus is on the puzzle, not monetization
- 5 languages — English, Japanese, Chinese, Korean, Turkish
The game engine runs on Phaser 3 with Canvas rendering. Lines snap to grid edges with clear visual feedback. Completed loops highlight in green. Invalid states show warnings.
For the Technically Curious
The entire platform runs on Cloudflare's free tier:
- Frontend: Next.js static export on Cloudflare Pages (global CDN, < 50ms TTFB)
- API: Cloudflare Worker handling puzzle serving, auth, game completion, leaderboards
- Database: Cloudflare D1 (SQLite at the edge) storing all puzzles and user data
- Puzzle Engine: Custom Rust binary for batch generation, compiles to WASM for browser-side validation
Monthly hosting cost: $0. The entire 3000-puzzle database is 1.9MB in D1.
How Difficulty Rating Works
Most puzzle sites rate difficulty by counting missing clues. That's lazy and inaccurate. Our Rust engine simulates a human solver:
- Apply the simplest technique (corner rules). If it makes progress, stay at Level 1.
- If stuck, try the next technique (adjacent patterns). If needed, bump to Level 3.
- Continue through vertex rules (Level 5), loop closure (Level 7), and bifurcation (Level 9).
- The highest technique needed = the puzzle's difficulty level.
This means a Level 3 puzzle genuinely requires intermediate techniques. A Level 7 puzzle will force you to use loop closure logic — guaranteed.
Getting Started
New to Slitherlink? Start with the corner patterns guide — learn the four corner rules (0, 1, 2, 3 in corners) and you can solve any Level 1-2 puzzle.
Ready to play? Pick 5×5, Easy difficulty, and go: slitherlinks.com
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