Wiktionary
n. (context chiefly northern US and Canada English) A walled structure constructed of snow and used for children's wintertime play.
Wikipedia
A snow fort or snow castle is a usually open-topped temporary structure made of snow walls that is usually used for recreational purposes. Snow forts are generally built by children as a playground game or winter pastime and are used as defensive structures in snowball fights. They are also built and used for make-believe games such as "house", "store", or "community", a game where multiple forts are built in a group. Along with the snowman, it is one of the two structures commonly built by children out of snow.
Snow structures made for sleeping are called igloos when made from snow blocks and quinzhees when made by hollowing out a pile of snow.
Usage examples of "snow fort".
Teevan that Kurt had taken a party of younger boys some distance into the woods to build a snow fort.
They'd decided this was going to be the snow fort to end all snow forts.
The Early Settlers built a snow fort on the hill, and a strong fortress it was, constructed of snowballs, rolled up to a vast size (larger than the cyclopean blocks of stone which form the ancient Etruscan walls in Italy), piled one upon another, and the whole cemented by pouring on water which froze and made the walls solid.
When the neighborhood boys were building a snow fort, for instance, if he built sloppily and carelessly like they did, there was no punishment.