Las Noches (虚夜宮（ラス・ノーチェス）, Rasu Nōchesu; Spanish for "The Nights", Japanese for "Hollow Night Palace") is the fortress located in Hueco Mundo and base of operations of Kinenbuke after their defection from Soul Society and the Royal Guard. They reside there with their army of vizard complete with medical facilities and detention wings. This was also Barragan Luisenbarn's castle when he was still the King of Hueco Mundo. However, during Barragan's reign it had no roof or walls. Indeed, Barragan scorned at the idea of a roof claiming that as he was the King of Hueco Mundo and as such the sky of Hueco Mundo was his ceiling. It was also once the base of Sosuke Aizen and his Espada. It has been updated and rebuilt by it's new inhabitants.
Las Noches is a massive fortress which can be seen for miles in Hueco Mundo. It consists of a main building with a domed center surrounded by several large towers and smaller buildings. Atop the dome are six smaller towers (or as they appear), which seem to contain areas such as the Great Vizard Library, which was once the area that contained Aizen's throne room. These "towers" also contain the Eleven Squads' meeting hall, once the Espada meeting hall. Also, in the room where the captains meet , there is one large rug on the ground that is has a cross on it, much like those crosses in the Christian faith. Like everything else in Hueco Mundo, the main building is colored white.
Las Noches is easily the largest building structure in the Bleach universe, appearing as large as (if not larger than) Seireitei in Soul Society. Due to its immense size, Ichigo Kurosaki once commented that it could be an illusion as he, Chad, and Uryū Ishida did not seem to make any progress getting closer to it, despite running until they were exhausted. Also, according to the Arrancar Nelliel Tu Oderschvank, it takes about three days to move from one spot to a gate.
The inside of Las Noches' dome simulate the living world and Soul Society's blue sky. Though because Ichigo was once able to break through the dome and escape to fight Ulquiorra, its more than likely that the area under the dome is like Urahara's training ground, due to the sky and seemingly endless plains. The position of the halls within the palace can also be adjusted, allowing the one operating them to direct the movements of intruders.
It's new features include; The outside walls of Las Noches are now made of Sekkiseki (殺気石, spirit reducing stone), a rare type of stone known to negate all Reiryoku, similar to magnetic lodestones, which prevents the ability to gather Reishi underneath your feet to walk in the air whist around Las Noches. The stone also forms a spherical barrier around the fortress, extending above and below, stopping anything spiritual from breaching it from ground level, from the sky or from underground.
It also contains the Great Vizard Library, which is a library for anyone in the Kinenbuke or who is a vizard seeking knowledge, but used in majority by the Kinenbuke themselves. Its purpose was simply to serve as the largest collection of known information in the world. Its founder, Shuk'la Verd, believed that any and all forms of knowledge would be used as a potential benefit for the Kinenbuke. Information was stored on thousands upon thousands of scrolls and in books.
The last of it's new features is an underground chamber which is the same size as the above ground fortress. The "ground" of this chamber is made of sekkiseki, but is covered by desert sands so that it is unnoticeable. It resembles the Menos Forest in the outside Hueco Mundo with small huts and spires that are used as living quarters and meeting halls for the Espada, who assist the Kinenbuke in their goals. It also has a hidden spire, disguised as one of the quartz trees of Hueco Mundo, that connects to the upper Las Noches and one secret passage, that leads to the dunes of Hueco Mundo.