The pact of three Races

[Lord Highland]

There is an idea that the Last Hero is also the Night's King. He had a company of a dozen men with him, and survived them all, just like the Night's King was the 13th Lord Commander.

We don't know almost anything about the Wall, but it is assumed that Bran the Builder built it with the help of the CotF to keep the Others away. The thing is, the Wall is made of Ice, which the Others are made of, and maybe even control. Plus, the CotF who are supposed to have helped, stayed on the Other side (pun).

The way I see it, there are two most probable scenarios:

1- The Wall was built by all three races to end the war among them, sealed by a Pact (I'm not referring to the one giving the CotF the forests, etc., this is a different one) signed by all three, probably splitting humans from the other two, and was guarded by men, Others & CotF magic. The Night's King, being a man at day and an Other (or was it Wight?) by night was the only Lord Commander acceptable to both sides. After some years though, the Stark Lord either figured out (or maybe they had it figured before but needed a few years of peace to produce more) how to use dragonglass, or discovered some other way to defeat the Others and drove them away. The Other were defeated and the Pact remained valid only between humans and the CotF.

2- The CotF really helped humans deafeat the Others but then they built a Wall beyond which they, and all other magical beings living in Westeros before the First Men arrived, could live undisturbed.

I like the first scenario better.

[falcotron]

See this thread, about why the Children live north of the Wall. But briefly, the problem you're trying to explain doesn't actually exist, so neither of your two scenarios works. The Children did not stay on the other side of the Wall. Every time we're told what happened to them, we're told that they gradually retreated deeper into the forests and the other lands untouched by men. Thousands of years later--as records and legends agree--the Andals arrived, and destroyed most of those forests. We know, for example, that there were still Children in High Heart before Erreg the Kinslayer killed them.

Also, the idea that a Stark Lord figured out how to use dragonglass to defeat the Others long after the War doesn't fit with anything we know. The Children have always used dragonglass. The Children taught the humans that dragonglass can kill the Others. The Children continued giving the Watch 100 obsidian daggers every year for millennia after the Wall was built.