This War of Mine: The Little Ones

In order to unlock this achievement, everyone in your house must survive the winter. If anyone dies for any reason, be it soldiers or illness, you will need to play again to unlock this achievement. Also, if someone leaves the house, it will void this achievement. The reason for this is that the danger from winter is dying from illness. When this happens, they don't die in the home, they just go out for a walk like Lawrence Oates (look it up). So, if you send a character out to help a neighbor, the game doesn't distinguish why the character is no longer available, it still invalidates the achievement. You can go out and scavenge, but you cannot send someone to help a neighbor.

You will get a little warning that winter is coming, either from reports on the radio or from temperatures around 5 degrees Celsius. If you are playing a custom game, you can control how long and harsh the winter is. A short winter will last for a few days (about 10% of the game), while a normal winter will last for twice that. And a mild winter will only see temperatures dip down to about -1 or so, while a normal winter will get below -5 degrees Celsius.

The key to surviving winter will be to have at least one furnace and some meds. Herbal meds won't necessarily suffice, and a standard furnace will suffice for a mild winter, but for anything worse, you will need at least one, if not two upgraded furnaces. Keep the furnaces full of fuel for maximum effect. So long as you are able to keep the temperature above 10 degrees Celsius, you should be fine, but above 15 is best. The achievement will unlock once winter ends (signified by the day screen returning to the normal war torn city and the snow disappearing from the yard).