Oh, GRR Martin was building up to the Night King. There's a lot more mythos in the books -- apparently the "tree people" created the White Walkers to fight off the humans. But we don't get much of the tree people in this -- some but not a lot. While in the books there's about 100-300 pages devoted to them.

It's not exactly clear where Martin was headed in the books.
He was actually more interested in the mythos and world-building than he was in the plot.