Sudoku Puzzle — a grid you can trust, plus a second puzzle nobody expected

The challenge

Sudoku is the most attempted puzzle genre on mobile, and that is exactly what makes it hard to ship. The rules are fixed and universally known, so there is nothing to invent and nowhere to hide. Every decision that matters is about how the grid feels under a thumb, and players who have played Sudoku for thirty years notice a bad one instantly.

The first problem is the one most Sudoku apps quietly get wrong: puzzle generation. A grid that has more than one valid solution can be "solved" by guessing, and a player who guesses correctly still feels cheated, because the deduction they were trying to make was never actually available. A grid with no solution at all is worse. Both bugs are invisible until a careful player hits one, and then the app loses its credibility permanently.

The second problem is that difficulty in Sudoku is not a number you can set. Removing more clues does not reliably make a grid harder — it changes which solving techniques are required. A grid with 30 clues can be trivial and a grid with 32 can be brutal, depending entirely on whether progress needs a simple scan or a chain of eliminations. Six labelled difficulty levels are a promise to the player, and that promise has to hold on every single generated board.

The third problem is attrition. A beginner who taps a wrong number and watches the board reject it with no explanation closes the app and does not come back. A hard grid is supposed to be punishing; learning the interface is not.

And underneath all of it, a classic genre carries a visual expectation. Most Sudoku apps look like a spreadsheet. Looking like a spreadsheet is safe, and it is also the reason none of them feel like something you would open for pleasure.

Our approach

Generation was built to guarantee uniqueness rather than assume it. Every puzzle is produced with exactly one solution, and that property is verified before the board is ever shown. This is the foundation the whole app rests on: because the solution is unique, every cell is reachable by pure logic, and the app can honestly tell the player that no guessing is ever required. That single guarantee is what separates a Sudoku you can trust from one you merely tolerate.

Difficulty is expressed as six named tiers — Beginner, Easy, Medium, Hard, Expert and Extreme — so the ladder is legible before the player commits. The naming matters more than it looks: a player who picks "Expert" and gets a grid that falls apart in two minutes stops believing the labels, and once the labels are meaningless the progression is gone.

The notes system does the bookkeeping a pencil would. Switch notes on and you can write candidate numbers into any cell, and when a correct number is placed, that digit is automatically cleared from the notes in its row, column and 3×3 box. This is the single highest-value feature in the app. Maintaining candidate lists by hand is the tedious half of hard Sudoku, and automating the cleanup lets the player spend attention on deduction instead of erasing.

Reading the board was treated as a first-class problem. Tapping any cell lights up its row, its column, its 3×3 box and every matching number on the grid — four overlapping constraints made visible at once, which is precisely the information a human needs to spot an elimination. A counter at the top shows how many cells remain, each number key shows how many of that digit are still unplaced, and a key gets a green check once its digit is complete. Together these turn "what should I look at next" from a scanning chore into a glance.

Mistakes are handled with a cost rather than a wall. Three hearts per puzzle means a wrong number matters without ending the session, and Undo, Erase and Hint exist for the moments when a player is genuinely stuck rather than careless. Stars are awarded up to three per solved grid, scaled by mistakes, so accuracy is rewarded without making a completed puzzle feel like a failure.

Progress is tracked where a returning player will look for it: a timer, a best time per difficulty level, and a count of puzzles solved. The timer stops when the app loses focus, because punishing someone for answering a phone call is not difficulty.

Then there is the part that is not Sudoku at all. Find My Animal is a second, complete logic puzzle built into the same app — a constraint game where one animal must be placed in every row, every column and every coloured region, and no two animals may touch, not even diagonally. Boards run from 7×7 up to 12×12 across the same six difficulty tiers, every board has exactly one solution, and it is solvable by logic alone. Tiles cycle through mark-X, place-animal and clear on successive taps, so the whole interaction is one gesture. When a placement breaks a rule the animal turns red and the game states which rule was broken, which turns an error into a lesson instead of a dead end.

Adding a second mode was a deliberate retention decision. Sudoku is a focused, sometimes tiring activity, and a player who is burnt out on grids but still wants a logic puzzle previously had no reason to stay in the app. Nine animals, soft presentation and a different constraint shape give that player somewhere to go without leaving.

Visually the app rejects the spreadsheet convention. Colourful 3D tiles with soft glossy buttons, sparkles on a correct placement, a wave animation when a row, column or box completes, "Great!" combos for correct moves in sequence, and confetti on solve. Calm background music and gentle sound effects sit underneath, with sound, music and vibration each separately switchable — because the player who wants silence on a commute and the player who wants feedback at home are the same person at different hours. Numbers are set large and legible, sized for phones and tablets rather than scaled between them.

The result

A Sudoku that earns trust on the first hard grid. Because every puzzle is generated with exactly one solution and verified before it is served, the app can promise that logic alone is always enough — and that promise holds on every board, at every one of the six difficulty levels.

The notes system with automatic cleanup removes the bookkeeping that makes hard Sudoku tedious rather than challenging, and the highlight system makes the four constraints that govern any cell visible in a single tap. Together they shift the player's effort from scanning and erasing to actual deduction.

Find My Animal turns a single-genre app into two complete logic games sharing one difficulty ladder, which gives a player who is tired of grids a reason to stay rather than uninstall. Nine animals, boards up to 12×12, every one uniquely solvable.

The 3D presentation, completion animations and per-channel audio settings move a classic genre out of spreadsheet territory without touching the rules players already know.

Live on Google Play, free, with optional rewarded video for extra hints and erases.