The building's name is Core Temple.
The Central Order of the Knights is led by Mother Knight herself. It's the strongest amongst the five Orders and it holds the leading role in politics of the entire human race. Anne, Pray, Jill, Milo, May Zail and Kamad are Knights of this Order. The Knight trainees all train within this facility before they choose which order they will serve in.
Its headquarters are on Arin, built around the Core Temple. The defense and security measures are said to be the world's finest and it's been rated as the perfect fortress. Even if thousands of large antimatter, planet-bursting bombs were dropped on Arin and it was demolished, the Central Knights fortress would, in theory, keep its shape. It completely servers all ties to the outside world once it goes into defense mode. It also has a rarely used invisible mode that blocks light from escaping outwards [1]
In the Battle of Arin, the Beasts conquered the planet and were able to steal secret data held within Central Order. The facility was overrun in minutes by the Beasts, an act caught on satellite after the invisible mode was deactivated.
The Core Temple of Central Order was built on the space fortress Avalon from the Age of the Great War.