The idea

Most math games feel like timed quizzes: answer fast, move on, repeat.

I wanted to build something with a different texture. In Number Duel, arithmetic is not the homework you do before the fun starts. It is the move system.

You pick numbers to shape the board, limit your opponent, and set up future threats. The math is simple enough to play immediately, but the choices start to feel surprisingly tactical.

How it works

Each player has an active number from 0 to 9.

On a turn, the two active numbers combine to create a target. You claim the matching cell on a 5x5 board. Connect four cells in a row to win.

For example, in Sum Duel:

• I pick 9
• You pick 3
• The target becomes 12
• The player whose turn it is can claim a 12 cell

The interesting part is that your number does not only create your own move. It also controls what your opponent can do next.

That shared constraint is where the strategy lives.

Three modes are live

Sum Duel is the most approachable mode. Active numbers are added together, so the board is easy to read and the tactics are about timing, blocking, and forcing useful totals.

Product Duel makes the board spikier. Multiplication creates repeated targets, unreachable zones, and more dramatic tempo swings.

Fifteen Duel is a smaller puzzle mode using the numbers 1-9. First to claim three numbers that sum to 15 wins. The hidden trick: the eight winning triples are exactly the rows, columns, and diagonals of the 3x3 magic square, so the mode is secretly tic-tac-toe in disguise.

The hardest problem was onboarding

The game logic was not the hard part.

The hard part was getting the rules to click before a new player bounced.

I tried to make the first screen do most of the teaching:

• Put the board front and center
• Highlight legal cells so players do not need to calculate every option
• Keep the first match short with a 5x5 board and connect-four win condition
• Move deeper strategy notes out of the way until players actually want them

I am still tuning this. If you try the game, the feedback I care about most is:

At what moment did the rules click?

The stack

The app is built with:

• React + Vite for the browser client
• Cloudflare Workers for the API surface
• Durable Objects for real-time multiplayer rooms
• Cloudflare D1 for daily challenge results and leaderboards

Durable Objects turned out to be a nice fit for this shape of game.

Each room maps to a single object instance. WebSocket connections stay in memory, the room owns the authoritative state, and important state can be persisted without running a separate game server.

For a side project, that tradeoff has been pleasant: small deployment surface, real-time rooms, and no always-on server to babysit.

Try it

No signup, no ads, and it works on mobile:

https://numberduel.mathmindpuzzles.com/

Strategy notes:
https://numberduel.mathmindpuzzles.com/strategy/

Magic square explainer:
https://numberduel.mathmindpuzzles.com/math/magic-square-15/

Feedback is very welcome, especially on first-time clarity and whether the game feels more like a puzzle or a duel.