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.