So now we go back to our small and pitiful lives for another year or so...
The season finale was satisfying.
It also got me to thinking. SPOILERS AHEAD
So the way the Night King got the Wall down was with the dragon. What was his plan for getting through the Wall before he learned about the existence of the dragons and was able to resurrect one (which took place only a short while ago)? He's been marching towards the Wall for a long time - was he just going to wing it when he got there?
It made me wonder if the Night King has a touch of Bran's ability to see into the past/future. Don't forget that the Night King was able to see and touch Bran when Bran did his "second sight" thing a few seasons ago. As far as I can recall, he's the only one who's been able to directly interact with Bran (Ned Stark heard Bran briefly at the Tower in Dorne but shrugged it off).
So, if the Night King is able to use second sight himself, then maybe he was able to foresee that he would use the ice dragon at the Wall. Which meant that he knew he was going to be able to get his hands on one, and how. Which explains why he was able to get those huge chains - he made them ahead of time because he knew he'd need them to pull the dragon out after he killed it.
His plan from the get-go was that he was going to bring the Wall down with a dragon. He just hasn't been able to see far enough ahead to see what will ultimately happen (or maybe he has, and he realizes he has no choice but to follow his fate).
Anyways, that's my theory.