Chains, the part that's meant to feel good
Merging is the core of the game, and it's the part we most wanted to get right. When the tile you just dropped touches a tile of the same number — up, down, left, or right, but never diagonally — the two combine into one tile of double the value.
The good bit is what happens next. That new, bigger tile can immediately touch another match and merge again, and again: a chain reaction from a single drop. Drop a 64 onto a stack of 64, 128, 256 and watch it roll all the way up — 64 and 64 make 128, which meets the 128 to make 256, which meets the 256 to make 512. One tap, three merges.
Two things keep it fair and readable. Merges resolve in a fixed order — down first, then left, right, up — so the same board always plays out the same way, with no randomness in the cascade. And the score is simply the sum of what you make: that 64-into-512 chain is worth 128 plus 256 plus 512, which is 896, with nothing hidden. We tested that exact example down to the number, along with the priority order and the rule that only the tile you dropped drives the chain — tiles that happen to line up elsewhere wait their turn.
The rules now produce real, scorable play. The last piece of the engine is the one that makes it trustworthy: making the game identical every time you start from the same point.