The first Halo field spell in my set. Allow me to explain its abilities...
Machine ATK boost: ghosts and turrets are EXTREMELY powerful on Snowbound, so an ATK boost seemed sensible.
Face down DEF boost: this is because of the ease of hiding in the structure and ambushing foes. They attack, they now have less chance to succeed. Fairly simple.
No card position changing except by card effect: the whole place is covered with snow. Snow hampers movement. Besides, otherwise Jackals would be WAY too powerful here!