Why a wrong flag still turns green

A number in Dawnsweeper does two jobs. It is a Minesweeper clue, and it is a piece of wall.

The second job comes from the Creep. Every three minutes on Medium the Tide comes, and the Creep takes one layer of ground from every edge you have left open in the parts of the board you’ve played recently. It can’t cross a number whose mines are all accounted for. A 2 with both of its mines flagged is sealed. So is a 0 with no covered cell around it. Everything else is an edge, and edges get eaten.

That makes flagging the most important thing you do. In ordinary Minesweeper a flag is a note to yourself, and plenty of good players never place one. Here a flag is what turns a number into a wall, and walls are what let you leave a stretch of ground and come back to find it still yours.

The note that started it

The first builds of the board showed none of this. You had to count. The note that changed it read:

“When I place a flag, numbers around it show as green if they are ‘fulfilled’. So that I know I can click on them and clear the surrounding hidden tiles - and also I know that they won’t be eaten by the creep.”

So a fulfilled number now glows green and gets a solid double line on each side that faces covered ground. The double line is there so the cue doesn’t depend on colour alone, and the green is a teal that doesn’t match the green of an ordinary 2. Just before each Tide, the open edges that are about to go get a moving dashed amber band, for about two seconds. Zoom out to the map and every fulfilled edge is drawn as one unbroken green line: your wall, end to end, with the gaps in it obvious.

Green also tells you where you can chord. Tap a green number and the cells around it that aren’t flagged open at once. That’s the same rule as classic Minesweeper, made visible.

What “fulfilled” means

Here is the part that took a decision. The game knows where every mine is. You don’t. When you plant a flag, the game could check it and only turn the number green if the flag is right.

It doesn’t, because that would be a free mine detector. Plant a flag, watch whether the number goes green, move the flag if it doesn’t. Every number would become a yes-or-no question you could ask for nothing, and the deduction would be gone.

So green means apparently satisfied: the flags and craters next to the number add up to the number. If one of those flags is wrong, the number still goes green. It looks exactly like a wall.

It isn’t one. The Creep goes by the truth. When the Tide comes, it treats a number with a wrong flag beside it as an open edge, because one of its mines is still unaccounted for. The Creep rolls over it, and when it re-covers a cell next to your wrong flag, the flag breaks. It’s gone from the board, and your streak resets, the same as a mine hit.

That’s the price of a guess, and you see it at the moment the Creep proves you wrong.

The same rule everywhere

The game keeps two pictures of the board: what you can know, and the truth. Everything it draws or plays comes from the first one. No sound, no colour and no animation tells you a flag is wrong before the board itself does. The green glow, the amber band and the map’s wall line all come from what you can see.

The one place the truth shows through is the Creep, because the Creep is the test. A chord through a wrong flag is the other: tap a green number whose flag is wrong, and the chord opens the real mine. That counts as a mine hit, the ground bursts, and every wrong flag beside that number breaks.

Settlers go by the truth too, when they judge which of your flags are safe to dig up. To them, a number with a wrong flag beside it is an open edge, and away from their buildings they leave the flags near it alone.

Why we didn’t make it easier

We could have made wrong flags safer. A softer game would let the Creep pass a wrong flag without breaking it, or mark suspect flags after a while. Every version of that leaks the answer.

The honest version is more interesting to play. A green edge you built by deduction is solid, and you know it. A green edge with a guess in it is a bet, and the next Tide settles it.