A box-pushing puzzle game with 500 levels. The interesting part is not on screen — it is how the levels were made.
A custom generator builds each level by starting from the solved state and walking the state space backwards. Pulling a box is the exact inverse of pushing it, so every state reached is solvable by construction — and its depth in the search is the minimum number of pushes. The optimal solution comes for free.
That makes it possible to measure difficulty by what actually causes it: traps, the legal moves that leave a level unsolvable without you noticing. Replacing solution length with that count raised the correlation with perceived difficulty from 0.47 to 0.96.
Mechanics: typed boxes that only count on a target bearing the same mark, and one-way tiles. Each difficulty band introduces exactly one new thing.
Accessibility: every piece on the board is distinguished by shape as well as colour — the whole game is playable in a black-and-white screenshot.