Game of Thrones favorite Samwell Tarly decided to go his own way during Seasons 5 and 6, despite fans viewing a merging of characters storylines. The world of Planetos contracted back down, heading towards the story's finale. Sansa and Jon Snow were finding each other, Tyrion was hooking up with Dany's crew, but Samwell left for bigger and better things.
Some responded to the events of Hardhome by choosing to stay and fight. Sam decided to listen to his beloved Maester Aemon and get Gilly and Little Sam as far south as he could, away from the oncoming terror. He convinced Jon in the Season 5 finale to let them go to Oldtown and the Citadel, where he could train as a maester.
How south is Oldtown? It's so far south, it's practically in Dorne. Technically it's just north of the border, in the area known as the Reach, which is ruled by Highgarden. It is a huge port city, which explains why one of its monuments is the Hightower, basically an enormous lighthouse.
Oldtown is notable for being the oldest city in Westeros and for having the two hearts of religious orders houses within its walls. First is the Starry Sept of the Faith of the Seven, which was the seat of power for the High Septon until the Great Sept of Baelor was built by Baelor "The Blessed" in King's Landing during his decade long reign in the 160s. The second is the Citadel, called "The Greatest Seat of Knowledge In The World." It is where those who wish to become maesters study to forge their chains.
Sam, at the end of Season 6, arrived just as the Citadel sent out the white ravens to announce the turning of the seasons, and that winter had arrived. On his way, he stopped at his father's house (also located in the reach) with hopes of dropping Gilly off, but instead wound up stealing his father's valyrian sword and heading out again.
So why is being in the Citadel so important? One reason: The maesters have records of everything. Everything.
They have records of things people once knew and forgot about, like how to defeat white walkers, the prophecy of Azor Ahai and other facts that might be very useful when the Wall falls. They have records of things people have no idea they need to know about, like how Dragonstone is seated on a massive storage of dragonglass, or that there are magical ways to travel and communicate through obsidian candles. (We should also mention that the Citadel is rumored to have a huge secret stash of magical objects that no one knows much about, which could also be very useful.)
It also records things that might have been more well-known if there hadn't been massive wars going on. Like, oh, say, Rhaegar annulling his marriage to Elia Martell in the middle of Robert's Rebellion and marrying Lyanna Stark, legitimizing their unborn son, who happened to grow up to be Jon Snow.
Ok, we don't actually know yet if the last part is a thing or not. That's just speculation. But! If it happened, Sam is in the place that would have that record. In short, Sam is is in the seat of all knowledge. And though Cersei might deny it, knowledge is power.
Will Sam find the knowledge that Westeros needs to defeat the Night King's army when it comes over the Wall? Hopefully he'll do it quickly as there's only seven episodes this season.
Game of Thrones' first episode of Season 7 debuts Sunday, July 16, at 9 p.m. ET on HBO.