since the coming winter is supposed to be epic in nature, and regular winters bring snows that are 100 feet deep to the North, it's also possible that the snows could top the wall...
It also strikes me that there must be more to the story of the Wall holding back the Others, because in the dead of winter, it shouldn't be too hard for an undead force to fross the (near?) frozen waters of either the Bay of Ice in the west (past Westwatch-by-the-Brige - westernmost fort on the Wall) or the Shivering Sea in the east (near Eastwatch-by-the-Sea - easternmost fort). The Others are definitely showing intelligence, and it's not like they need to worry about supply chains.