Fenestrated walls, also known as fenestrated planes, are used as a kind of video surveillance utilized in Derse. They consist of a wall panel made up of four separate windows used to show events elsewhere - these walls can be broken, and entered. Four fenestrated walls are arranged in a square to form a "Cubicle of Vigilance".
The Archargent Jack Noir's fourth fenestrated wall was stolen, presumably by Grandpa, much to his intense annoyance. The fourth wall was later located (and broken) to show MSPA author Andrew Hussie for a shameless self-insertion into the story as well as a verbose recap of events to that point. The fourth wall was then taken by Jade as her future self advised via Karkat. She then used it to escape the scratched session with John, Skaia, and the kids' four planets.
Roxy Lalonde has a number of fenestrated walls in her room. Unlike earlier walls, many of these (like the windows of post-scratch houses) have five panes: four small and one large. All of Roxy's fenestrated walls are black, except for one that appears to be a window, which may be a reference to the Windows in Problem Sleuth's, Ace Dick's, and Pickle Inspector's offices in Problem Sleuth. One of the others is connected to a Skaianet terminal. This apparently relates to her skill in the esoteric science of DARK FENESTROLOGY.
Each of these Fenestrated Planes is connected to another one elsewhere in Roxy's house - anyone who jumps in at one end pops out of the other. Although from the outside travel seems instantaneous, the person going through the planes experiences time spent falling in voidspace (later revealed to be the Furthest Ring) varying depending on the distance between planes. From the ring, the window pane is glowing brightly. Turning a plane off breaks the connection, which causes the end of the window pane in the Furthest Ring to grow dark and seemingly become solid ground when someone lands on it. Much like the Windows of Problem Sleuth, strange things happen to anyone caught in voidspace when the window lights go out. When Roxy is trapped between window panes (thanks to Gcat), the strange environment of a dream bubble appears around her, where Roxy immediately encounters Meenah.
A wall almost identical to the ones Roxy has (but with a white frame and panes) was seen in Jade Harley's room here.
. She is said to have built it based off the design of the Dersite fenestrated wall in Grandpa's possession, but hasn't been able to make it work. She doesn't seem to even understand what it does (at the time of her introduction, anyway), and it is only referred to as a "broad, flat gizmo," "one of [her] GRANDPA'S more mysterious inventions."
Jack's Cubicle of Vigilance in the post-scratch session is also missing its fourth wall, which was stolen by Grandma, matching its fate in the pre-scratch universe. It was later shown to be located in the top room of Jake's old house, which he kept in his sylladex. He eventually had Terry Kiser shrink the house before retrieving the fenestrated wall. This fenestrated wall served as John and Jade's entrance to the post-scratch session. The other three walls were later destroyed by Dirk Strider to prevent Derse's agents from locating him.