In Chapter 21 ("XXI,") as the men make their way to the cell known as "One Hundred and Five, North Tower," the mood is tense and spooky. They enter the room, and here's the narrator's direct description of the cell and of what the men discover there:
There was a small, heavily-grated, unglazed window high in the wall, with a stone screen before it, so that the sky could be only seen by stooping low and looking up. There was a small chimney, heavily barred across, a few feet within. There was a heap of old feathery wood ashes on the hearth. There were a stool, and table, and a straw bed. There were the four blackened walls, and a rusted iron ring in one of them.
“Pass that torch slowly along these walls, that I may see them,” said Defarge to the turnkey.
The man obeyed, and Defarge followed the light closely with his eyes.
“Stop!—Look here, Jacques!”
“A. M.!” croaked Jacques Three as he read greedily.
“Alexandre Manette,” said Defarge in his ear, following the letters with his swart forefinger, deeply engrained with gunpowder. “And here he wrote ‘a poor physician.’ And it was he, without doubt, who scratched a calendar on this stone...”
So what we know about the cell known as the 105 North Tower is, for certain, that it's a creepy place where someone has been held captive for some time. The most interesting thing about the room is the marks left behind by the prisoner: there were the initials carved into the wall ("A. M.") and the phrase carved below that was "a poor physician." There was also a calendar that was scratched onto a stone, which makes sense: someone imprisoned would be interested in keeping track of the days as they go by, especially if he had nothing else to do.
Beyond those interesting features of the cell, we also learn from the narration that it's a claustrophobic kind of place. There's only one tiny window, and you have to scoot yourself into an awkward position to see out of it; there's a stone screen blocking your view, and all you can see is the sky anyway. There's a fireplace, which kind of sounds nice, but this one is barred up and all ashy. There's furniture, but it's really sparse and basic: just a table and a stool, plus a bed made out of straw (really cheap, really uncomfortable material). And finally, all four of the walls are blackened. Taken together, these details convey an image of a creepy prison cell.