Number Kingdom: Defend the Castle

From NumWar to Number Kingdom

Number Kingdom started as a set of tabletop rules you can play with playing cards and blocks (the prototype was called "NumWar"). The order isn't an accident — I wanted to confirm children actually want to play before spending effort on a digital build. Once confirmed, what shipped was the warmer, more family-friendly Number Kingdom: the same math core, now a castle to defend.

The core loop

Rock-paper-scissors → decide who acts → choose attack / build / defend → attack & defend the two castles → resolve

• Rock-paper-scissors gives a zero-cost fair start and tension.
• Defending the castle turns addition and subtraction into a visible, satisfying push-and-pull — build up your own walls, chip away at theirs with mental math (see the article: Subtraction isn't 10−7).
• The three-way choice provides just enough strategic tension.
• Short rounds + comebacks drop the barrier to "one more round" to a minimum.

What it's validating

Number Kingdom is currently in Stage 1 — validation. The only goal isn't to look good, isn't to ship to the store, but to answer one question:

Can it get children to play repeatedly, on their own, and spontaneously count, compare, add, and subtract along the way?

The full design of that validation is recorded in EXP-001.

How to play it

The digital version is in TestFlight beta and self-testing; the physical rules are ready to use. To join the beta, or to get the physical rules, find me on Xiaohongshu.

The north star is just three words: one more round.