How did Samwell and Gilly get back over the wall?
During the course of Game of Thrones, under interrogation by Mance Rayder, Jon Snow confesses that only three of the wall's keeps are manned, and so we know that the Nightfort is unoccupied. Samwell corroborates this:
"We're west of Castle Black, but the Nightfort's closest to us. It was the first castle on the Wall. The Watch abandoned it during the reign of King Jaehaerys the First. Too big, impossible to maintain."
Without someone to open a tunnel door (if the Nightfort even has one), how did Samwell and Gilly manage to get over the other side of the wall?
It's incredibly unlikely they climbed it like the Wildlings, so how else could they have gotten inside the Nightfort?
I'm looking for a TV-continuity answer, not a book-continuity answer: although if one is available from the book, please spoiler-box it if it contains sensitive plot material.
Best Answer
As Sam explained in S03E09, because of his reading of some books at Castle Black, he knew of a secret passage through the Wall to one of the abandoned Night's Watch outposts, Nightsfort.
[The Nightfort]'s got a secret sally port, the Black Gate, as old as the wall itself. No one's used it in centuries, most likely. It leads through the wall to the Nightfort.
In S03E10, when Sam, Gilly, Bran, Hodor, etc. all meet, that's in Nightfort, on the south side of the Wall.
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Game of Thrones S03E10 - Sam and Gilly run into Bran's group
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Answer 2
I don't think it's ever been explained in the tv series. The book explanation isn't remotely spoilerific, but I'll tag it just in case. From Chapter 56 (Bran) from a Storm of Swords:
They use the Black Gate. It is a hidden gate that allows passage to the other side of the wall. It's actually set in the wall and looks like the old face of a weirwood tree (old, pale, shrunken, wrinkled). For an idea, see this image. Effectively, when someone gets near the gate its eyes open and it asks who is there. The person must repeat the vow of the Night's Watch, at which point the mouth of the wall opens until it is wide enough for people to pass. This is how Samwell and Gilly return.
Edit: As @Keen's answer shows, I stand corrected as there is some explanation in the TV series. The book though, as usual with GoT, does contain more information on these things.
Answer 3
After the White Walkers attack Sam, Gilly & Gilly's baby at the abandoned hut, which they set fire to in attempt to kill the White Walkers, they meet Coldhands riding a horse. We never really find out who or what Coldhands is, although we suspect he was a ranger that was turned by a Wight Walker but somehow retains consciousness and independent will. Coldhands leads Sam, Gilly & baby to a secret passage within the Wall. It is still protected by magic, thus preventing Coldhands from entering as well.
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