A magic tower in the city of Qarth that Dany went to in Season 2 to find her dragons. She ended up having a vision where she sees the throne room covered in either ash or snow, and then bypasses the throne to head to the wall.
That is why some people theorize that she will never sit on the Iron Throne.
Basically. Cats have literally been talking about this scene (HofU) for years yo
Ever since Hardhome where we saw how powerful the WWs were, the theories started flying about the White Walkers destroying King's Landing and making Dany's vision come true. This shyt is years old. Now the second part of the dream with Drogo's tent being behind the Wall, I didn't understand that fully at first years ago but, now it makes sense. Because we've seen Hardhome, we've seen the Walkers turn babies into demons, we've seen Hodor and the others die fighting them, we've seen Jon tell anyone who would listen that this is the only war that matters, we've seen Jon tell Dany in the cave that the Walkers are real. It's like the Salem Witch Trails in here. Everybody thinks spoilers are being posted
None of this stuff is spoilers. It's just details nikkas probably missed or forgot.