The Guild is an organization that specializes in making artillery weapons and ammunition. They are affiliated with the SSS and they act as the primary supplier of weapons for them. Chaa is the leader of the Guild.
Their headquarters are located deep underground, divided into twenty-one levels protected by a variety of traps and security systems. Yuri initiates Operation Descent to allow members of Guild to disable the security systems.
The deepest point is the main headquarters, which resembles a subterranean Steampunk city filled with pipes and machines. In this location, workers who are, like the SSS, students, make weapons out of dirt. They are able to do so based on their memories of weapons in the living world, and the weapons they make would fail catastrophically if they do not have any memories of seeing such a weapon.
Because of an unexpected infiltration by Angel, Guild is forced to destroy their headquarters--and Angel with it-- and headed for Old Guild, a former base.
In Episode 8, Angel was held captive by her Harmonics clones in the Guild after it was destroyed.