Scout describes the physical characteristics in detail, and it apparently wasn't a pretty place.
The Maycomb jail was the most venerable and hideous of the county's buildings... a miniature Gothic joke.
While most of Maycomb's buildings were "square-faced and steep-roofed," the jail had "tiny battlements and flying buttresses"--a much smaller version of a castle. It was only one cell wide and two cells high,
... on a lonely hill... wedged between Tyndal's Hardware Store and The Maycomb Tribune office.
It was absolutely the most distinctive building in the town.
When Scout appears at the jail on the night that the lynch mob tried to take Tom, she didn't fully understand why the men were there. She had just witnessed another "gang" of people meeting with Atticus in his front yard; he had assured her that the group came in peace, and Scout probably figured the lynch mob came to talk with Atticus--not to hurt him. When she ran into his arms, she saw the fear in his face, however, and soon recognized that the men were all strangers. Scout and Dill stood by Jem when he refused Atticus' orders to leave, and when one of the men grabbed Jem,
I kicked the man swiftly... I was surprised to see him fall back in real pain. I intended to kick his shin, but aimed too high.
Scout did not understand the severity of the situation, but she quickly forgot about Atticus' pained expression when she recognized Mr. Cunningham. Scout, who went to school with Mr. Cunningham's son, began making small talk--about Walter Jr., school and entailments. Her only anxiety came when she began to sweat.
I could stand anything but a bunch of people looking at me.
Scout was still clueless about the men's true intentions until Atticus discussed the matter with his sister the next day.
"So, it took an eight-year-old child to bring 'em to their senses, didn't it?" said Atticus. "That proves something--that a gang of wild animals can be stopped, simply because they're still human. Hmp, maybe we need a police force of children..."
Scout had made Mr. Cunningham stand in Atticus' shoes, and he realized that their decision to hang Tom was a bad one.