Las Noches (Spanish: "The Nights") is a massive fortress which can be seen for miles. It consists of a main building with a domed center surrounded by several large towers and smaller buildings. Atop the dome are five smaller towers, which appear to contain areas such as a throne room, detention cells, and the Espada meeting hall. Like everything else in Hueco Mundo, the main building is colored white. Las Noches is easily the largest known structure in existence, appearing as large as (if not larger than) Seireitei in Soul Society. Due to its immense size, some believe it could be an illusion, as it can be easily seen from a distance, and appear to get no bigger even after hours of walking. According to some, it takes about three days to move from one spot to a gate.

The inside of Las Noches' dome simulates the blue sky of the living world and Soul Society, using artificial light and painted walls and ceiling. Inside the dome, several large buildings can be found, including the palaces of the Espada, as each of them is given their own living space, which they can use as they see fit, and house themselves and their Fraccion. The position of the halls within the palace can also be adjusted, allowing the one operating them to direct the movements of intruders.