Celestial Sudoku Guide

What Celestial Sudoku is

Celestial Sudoku is a web sudoku game at sudoku.liuzhaohan.com. Every board is the standard 9×9 grid, and the goal is to fill every empty cell with 1 to 9 according to the mode's rules.

• Free, no download: open it in a browser and play. You can also install it to your phone or computer's home screen and open it full screen like an app.
• Devices: modern browsers such as Chrome, Edge, Safari and Firefox on iPhone, iPad, Android, Windows, macOS and Linux. Landscape and portrait each have their own layout.
• Input: tap or click a cell, then a number on the pad. On a computer you can play entirely from the keyboard.
• Languages: Simplified Chinese and English, switched from the top-right of the home screen or in Settings.
• No sign-up needed: guests can play every solo mode, the daily challenge and friend battles. Signing in adds synced results, leaderboards and friends.
• Pure logic: every puzzle has exactly one solution and is verified to need no guessing.

The four sudoku modes and their rules

All four modes share one board and one set of tools. Classic is the base; the other three are twists on its rules. The mode card on the home screen switches between them, and its picker lists what gets easier and harder than Classic in each.

Classic sudoku

Every row, every column and every 3×3 box holds the digits 1–9 exactly once.

The recommended place to start.

Jigsaw sudoku (irregular sudoku)

Nine irregular shapes outlined in bold replace the 3×3 boxes. Each shape still has nine cells holding 1–9 once; rows and columns work as in Classic.

No new techniques, but odd shapes are harder to scan.

Diagonal sudoku (Sudoku X)

On top of the Classic rules, both long diagonals (top-left to bottom-right and top-right to bottom-left) must also hold 1–9 once. The centre cell belongs to both.

The diagonals add clues, but every entry has two more lines to check.

Killer sudoku

Dashed cages split the board, each with a total in its corner. The digits in a cage add up to that total and never repeat inside the cage; the Classic row, column and box rules still apply.

There are almost no starting digits, so you reason with sums and combinations.

Difficulty levels

Every mode has Easy, Medium, Hard and Expert; Medium is the default. Difficulty is not just the number of givens. The server generates each puzzle, checks that it has a unique solution, then solves it with a human-style logic solver (singles, locked candidates, pairs, X-Wing and more) and rates it by the reasoning workload. Puzzles that land in the wrong band or would need guessing are thrown away.

Expert puzzles have the fewest givens and the most reasoning, and some of them require advanced eliminations such as locked candidates, pairs or X-Wing.

Controls

Touch and mouse

1. Select an empty cell. Its row, column, box and matching digits are highlighted (you can turn this off in Settings).
2. Tap 1–9 on the number pad. In landscape the pad sits on the right; in portrait it sits below the board.
3. The tools are Candidates (notes mode), Delete, Undo and Hint. Solo games can be paused at any time, which blurs the board.

Keyboard shortcuts

The Rules button in every game repeats the mode's rules and the shortcuts. Rapid input is applied in order and never dropped.

Notes, hints and assists

• No error marking: entries are never judged, turned red or counted as mistakes, and there is no mistake limit. Every deduction is yours.
• Candidates (notes): with Candidates on, numbers are written as small pencil marks. Hold Candidates for two seconds to fill every empty cell's candidates from the current board (Killer also uses cage combinations); it can be undone.
• Hints: three per game. A hint reveals the selected cell's correct digit and locks it. Givens and cells you already got right cost nothing. Hints cannot be undone.
• Deduction help: in solo games, select an empty cell and ask for deduction help from the Rules dialog. If the visible digits settle that cell directly (only one digit is left, or it is the last empty cell of a Killer cage), it walks you through the observation, reasoning and conclusion; otherwise it suggests noting candidates or looking at another cell. It never uses a hint or changes the board.
• Clear board: hold Delete for two seconds and release to clear all entries and notes (givens and hinted cells stay). It can be undone.
• Optional assists (Settings › Gameplay, with Beginner / Experienced / Custom presets): related-cell highlights, matching-digit highlights, completed units, remaining digit counts, automatic note cleanup and hold-to-fill candidates.

The completed-unit highlight only means a row, column or box is full with no repeats. It does not mean the digits are correct.

Daily challenge

• Each day has a four-stage route, Easy → Medium → Hard → Expert. Classic, Jigsaw, Diagonal and Killer each appear once, in an order set by the date.
• Everyone gets the same puzzles that day. Stages unlock in order, each stage has its own three hints, and you can continue a stage later from the Daily page.
• Days follow UTC: the challenge resets at 00:00 UTC, and past days cannot be made up.
• The page shows your current and best streak. Finishing all four stages shows your total time, a countdown to the next day and a link to today's leaderboard.

Friend battles

A friend battle is a live two-player race: both players get the same puzzle at the same moment, and the first correct solution wins.

1. On the home screen choose Online battle, select Create room, and set the mode, difficulty and whether hints are allowed.
2. Send your friend the six-character room code or the copied invite link. The link fills in the code automatically, and the Join field also accepts a whole pasted invite message. Signed-in players can invite online friends directly.
3. Your friend reviews the rules and presses I’m ready; the host presses Start battle. The host can change the rules while waiting, and a real change asks the friend to ready again.
4. During the race you see your opponent's nickname, avatar, connection and filled-cell progress (filled, not correct, so it never leaks answers). Their digits and notes stay private.

• Rooms hold two players, and no account is needed. Battles cannot be paused. When the host allows hints, each player has three.
• Forfeiting asks for confirmation and counts as a win for the opponent.
• A reload or a short drop reconnects you to the same room. A player who is offline for a total of 2 minutes while the opponent stays connected loses. If both are offline the clock does not count.
• Rooms are saved and restored when the site restarts for an update, and the downtime is not counted.
• Play again reopens the same room with a new puzzle. Signed-in players can move a room to another device.
• Two different signed-in players keep a head-to-head record (wins and win rate), and Records › Friend battle lists your latest 20 matches.

Leaderboards and score checks

There are three leaderboards:

• Fastest time: one board for each of the four modes × four difficulties, showing each player's best time. Ordinary games, daily stages and solved battles count; times with hints count too and show the hint count.
• Daily: today's daily challenge, ranked by stages finished (up to four) and then total time.
• Daily streak: the longest run of days with all four stages finished.

Only signed-in players are ranked, and boards show nicknames and avatars, never emails or account IDs. The server checks every result:

• puzzles and answers are issued encrypted by the server, so the browser cannot read the answer, and the submitted board must match the original puzzle;
• the time can be no longer than the real time the server measured from the start of the game to the submission (plus 5 seconds), and no shorter than 0.25 seconds per empty cell;
• hints are counted by the server, at most three per game, and replaying old data cannot reset them;
• only the first submission for a puzzle or daily stage is kept.

These checks stop forged times and unlimited hints, but they cannot prove who solved the puzzle. Signed-in players can report suspicious results, nicknames or avatars for an admin to review, and anyone can opt out of rankings in Settings while keeping their personal records.

Accounts, friends and your data

• Guests can play without an account; progress stays in that browser.
• Sign-in uses a six-digit email code (valid for ten minutes) or a Google account. Your first sign-in imports the guest games from that browser. Sessions last 30 days.
• What syncs: results, daily completions, tutorial progress and preferences. Unfinished games stay on the device.
• Nickname and avatar: nicknames are public and 1–12 characters long. Members can upload an avatar, which the server shrinks and strips of photo metadata.
• Friends: every member has an eight-character friend code. Add friends by code, from a battle result or from the leaderboard, see whether they are online, in a room or in a battle, and invite online friends into your room.
• Records: the Records page has Solo and Friend battle tabs with total games, play time, no-hint share, streaks, personal bests per mode and difficulty, and recent games.
• Privacy: you can turn off preference sync or leave the rankings, and Settings › Privacy exports all your data as JSON or permanently deletes your account.

Tutorials and advanced techniques

Learn Sudoku on the home screen is a three-tier path with bronze, silver and gold medals:

• Basics: one four-step lesson each for Classic, Jigsaw, Diagonal and Killer, ending with an independent exercise. The Classic lesson also teaches selecting, entering, deleting, undoing and candidates, and experienced players can skip that demonstration.
• Advanced: a six-step Classic course on hidden singles, locked candidates in both directions, naked and hidden pairs, and turning an elimination into a placement.
• Expert practice: fresh puzzles for X-Wing, Swordfish and XY-Wing, screened so simpler techniques cannot shortcut them. You can hide the structure marks and find the pattern yourself.

Lessons have no hint button: a wrong entry prompts you to check the rule, never shows the answer. Tutorials do not count toward records and never replace your current game.

Offline play and installing

Celestial Sudoku is an installable web app (PWA). After your first visit the whole game is stored on the device and opens without a connection:

• Offline, Start game generates the puzzle in your browser with the same engine and shows an "Offline" tag; hints still work.
• Offline games stay on that device and are not synced or ranked. Saved games, solo records, tutorials and settings all work offline.
• Friend battles, leaderboards, accounts and friends need a connection. Offline, their entries are dimmed and come back on their own when you reconnect.

How to install

• Chrome or Edge on a computer: click the install icon at the right of the address bar, or choose Install app in the browser menu. You can also press Install under Settings › Privacy › Install on this device.
• Android (Chrome): menu › Add to Home screen or Install app, or press Install in the game's settings.
• iPhone / iPad (Safari): tap Share › Add to Home Screen.

When a new version is ready the page says so and offers a reload, so a game in progress is never interrupted.

Settings and languages

• Display: light or dark theme, 简体中文 or English, and layout. Automatic picks the closest of 16:9, 16:10 and 4:3 landscape or 10:16, 9:16 and 3:4 portrait for your window; you can also lock one.
• Gameplay: Beginner (every assist on), Experienced (every assist off) or Custom, with a live sample board.
• Accessibility: larger digits, high contrast and reduced motion. The board works with screen readers and full keyboard control.
• Privacy (Data & Privacy): sign-in, sync and ranking switches, data export and deletion, install, and restoring defaults.

On your first visit the language follows your browser's preference; after that your own choice wins.