The mansion based in the Capital is the main dwelling of the Sacred Ancestor before his disappearance and traveling on the Frontier. Not many were invited to the mansion. He held small events of choice individuals on occasion such as for a song by his human wife who was pregnant with the twin Ds. Madame Laurencin was one such Noble in attendance.
He also entertained guests who had questions or issues with his policies laid down. Not many took up this open forum as anything he decreed was law with out consultation.
The exact location of the mansion isn't mentioned but the means to get there seemed to be common knowledge. It is not mentioned since the humans have taken control of the Capital them acquiring the means to travel to it or knowing its location. It is most likely extradimensional in nature and makes use of The Sacred Ancestor's Technology. Making it well beyond the understanding of most Nobility or perception of humans. He may have also made it inaccessible to anyone after he left.