Ezra's Cabin was seen throughout Season 4 as a place where Aria Montgomery and Ezra Fitz met up when they were keeping their relationship a secret from everyone. Ezra has a hidden door under the carpet that leads to the basement where he spied on the girls in addition to his secret lair in Ravenswood.
Ezra brings Aria to his cabin and he gives her the key, saying that it could be their secret. The two kiss. Later in the episode, Aria is seen gazing out the window of the cabin and Ezra looks at a trapdoor on the ground. Near the end of the episode, a figure, presumably A or Ezra, walks into the cottage and pulls open the trapdoor, revealing a hidden basement.
The Liars are stranded in the rain after Emily's car breaks down, and they go to Ezra's cabin while Aria pretends that it's her uncle's cabin. Ezra sneaks in and locks Spencer and Aria in the closet while Hanna and Emily are outside, and steals Alison's Diary.
He is seen spying on the girls in his basement with his surveillance equipment when he sends Aria out to the supermarket to buy more chickpeas.
Aria visits Ezra's cabin and finds his password to be B26, then finds his crime novel about Alison, which shocks her. She also finds his basement, but she doesn't see the giant computers.