Who Built Moat Cailin? (and Yeen?) Westeros Disaster Hunters - Ice and Fire Theory

Moat Cailin is a big creepy abandoned (and ruined ) megalithic fortress whose origins seem more than a tad fishy... the official story doesn't add up, so let's see if we can figure out what's really going on here, who built, and why. We'll also end up solving the Yeen mystery on the way, for you Yeen fans out there.