The Underground Assembly Hall is where the Central 46 meets. It is an octagonal structure located in a large open space within the main building, and was also referred to as the Diet Building. Beyond the entrance to this structure there is a staircase leading down into the hall. The benches and high-backed chairs of the 40 wise men are arranged in two concentric rings, one higher than the other, surrounding an open area. In the walls there are recesses with stairs leading up to them, where the six judges are seated. There are large banners on its walls. It has thirteen levels of protection to keep out or at least stall intruders. During Aizen's rebellion, the Assembly Hall was placed under lock down.

Notable events to occur in the Assembly Hall are the massacre of the Central 46 by Aizen, the manipulation of Rukia Kuchiki's execution, the trial of Kisuke Urahara, which took place 101 years before the start of the story, and the sentencing of Sōsuke Aizen. On this occasion, the members of the Central 46 sat behind numbered screens to maintain their anonymity. The Hall was kept in darkness during the trial, with the only lights shining upon Urahara and his co-accused, Tessai Tsukabishi.