A huge chamber with a high ceiling, it contained the Count's formal throne and desk, which were placed on a raised dais at the far end of the room. On the back of the throne stood a circular 22-meter-high glass window, etched with a sigil of House Serenno, through which the sunlight bathed the room with a greenish glow. Behind the desk, there was a secret turbolift that traveled throughout the main tower of Castle Serenno. The throne chamber was directly connected to the Count's sleeping chamber by a gangway.
Long before the Clone Wars, Count Dooku promised Asajj Ventress to teach her the dark side of the Force however she had to prove herself first.
In 20 BBY, Count Dooku contacted Darth Sidious from the hall where he was commanded to kill Ventress. Dooku obeyed his master. Later, Ventress contacted Count Dooku there during the Battle of Sullust where she asked for his help which he denied and instead ordered a Recusant-class light destroyer to destroy the command ship which she crashed inside the main hanger of. He then contacted Sidious, informing him that Ventress was dead, not knowing that she survived the battle.
Inside the great hall, Dooku fought against who he believed to be three Jedi that were actually Nightsister assassins wielding lightsabers, Ventress, Karis. Dooku won the fight and blasted the three attackers out of the castle that led to the window inside the hall being shattered.
Mother Talzin later contacted Dooku informing him that she may have a replacement for a Sith assassin with Ventress presumed dead. Dooku was interested in the offer that Mother Talzin made and left for the planet Dathomir to meet with her. Later, he met Savage Opress, who had been magically enhanced by the Nightsisters and took him on as his apprentice.
Later, Dooku trained Opress in the great hall, instructing him in lightsaber combat.