The Middle House is a evil location in The Keys to the Kingdom series. It is a giant, thrice-terraced mountain consisting of 3 large plateaux previously under the control of the so-called "lustful" (or mind-thristy, if you will) Lady Friday. It was the 5th demense that Arthur gained control of as the Rightful Heir.
The plateaux are connected by the Extremely Grand Canal, a waterway whose water transports any writing to its desired destination. The workers of the Canal wear clothes made of out of paper covered with writing so that they float if they fall into the water. Due to Friday's obsession with experiencing, it became thoroughly wintry.
Each of the plateaux, or “shelves”, serves a different function. The bottom, and the largest, called the Flat, is location of the Guild of Gilding and Illumination and of Letterer's Lark. The next shelf, the Middle of the Middle, houses the Guild of Illustration and Augmentation. Finally, the Top Shelf has the High Guild of Binding and Restoration. In between the Top Shelf and the crest is a crack in the mountain leading to the Eyrie of the Winged Servants of the Night, which included the former prison of the Fifth Part of the Will.
On the very crest of the mountain is Lady Friday’s Scriptorium, where she leaves a replica of her Key, in hopes of killing Arthur, the Piper, and Saturday. This trap, although it failed, opened onto the Void and managed to destroy the entire Scriptorium and a good portion of the mountain top; however Arthur, using all of the four Keys under his control, managed to stop the void but was unable to fill the gap it left behind.
At the end of Lady Friday, Arthur granted Dame Primus Stewardship of the Great Maze and the Middle House, although he retained the Fifth Key.
In Superior Saturday, the bulwark between the Middle House and the Lower House was weakening due to erosion caused by Nothing. The Lower House and Far Reaches had been destroyed when the Pit was breached and the two demenses were consumed by the Void of Nothing. Dame Primus is told that a geyser of Nothing has sprung nearby Letterer's Lark and, upon Arthur's insistence, splits into two parts, Dame Quarto and Dame Septum; one to deal with the bulwark of the Middle House, now the only protection of the Middle House against the Void, and the other to repair the mountain barrier of the Great Maze. Towards the end of Lord Sunday it is consumed by the void.
However, the Middle House was very like rebuild by the New Architect after the horror ends.