The Old Mansion is a large manor built on the outskirts of Twilight Town. It is mysterious and derelict - rumor has it to be haunted, or at least is said in the Simulated Twilight Town. Its past is an enigma.
 Settings and Areas
After leaving The Woods, you arrive at The Old Mansion which is the name for the gate and the front courtyard. Upon entering the Foyer, the Mansion splits off into three different directions. If you go into the door on the left side of the Foyer, you enter the Dining Room. If you go up the left set of stairs and follow the walkway to the door, you reach the White Room. If you go up the right set of stairs and enter the other door, you reach the Library.
In the Library, there is a false floor that hides a staircase that goes downward which leads to a door. Through that door is the Computer Room where Ansem the Wise was able to monitor the Simulated Twilight Town. There is a machine that also sends Roxas and Riku into the Simulated Twilight Town. The doorway to the left of the computer leads to the Basement Hall. The Basement Hall then leads to the Basement Corridor, and finally to the Pod Room.
In order to catch Roxas at the simulated Old Mansion, Axel creates a Corridor of Darkness from Dark City to the Basement Hall, but never closes it. When Sora and his friends return to the simulated Twilight Town, they are able to go through the portal and travel across the emptiness of Betwixt and Between to enter Dark City, and begin their assault on the Organization's stronghold.
When Roxas first comes to life, he meets Xemnas, who offers Roxas into Organization XIII. It's also the place where Xion visits Naminé asking about her origin, as well as the place where Xion and Axel fight each other. There are other times when Roxas must come to the Mansion gate for a mission.
During Sora's visit of Castle Oblivion, Vexen creates a memory version of Twilight Town, based on Roxas's memories of the place. After exploring the place, Sora turns up in the clearing outside of the mansion, where he and Vexen face off. Once Vexen is defeated, he begins ranting on about false memories and Sora being used as a puppet. To protect Roxas's identity and the interests of Marluxia, Axel shows up and assassinates Vexen before Sora's eyes. He leaves a confused Sora to make his way out of the city and back into Castle Oblivion. A short while later, Riku, also in Castle Oblivion, visits the memory Twilight Town thanks to King Mickey. After meeting DiZ, Riku and Riku Replica face off, with the Replica defeated once and for all. Riku then meets Naminé, who offers to put Riku into a deep slumber in order to seal away the darkness in his heart, to which he refuse.
 The Simulation
The Old Mansion first appears on the first day of Roxas's stay in the simulated Twilight Town. When a Dusk appears and steals the gang's picture of Roxas' success over Seifer (or if Roxas loses, Fuu's picture of Seifer's success over Roxas), Roxas chases after it through the streets of Twilight Town, and eventually into the forest on the edge of town. Finally ending up in the clearing in front of the mansion's locked gates, Roxas had a showdown with the Dusk, but his weak Struggle bat was useless against the Nobody. In order to save Roxas from being beaten down by his foe, DiZ equips him with a digitized Keyblade. Roxas then quickly dispatches the enemy, who does not stand a chance against the might of the weapon. The Keyblade disappears, and Roxas returns to the gang with the missing photos.
The mansion doesn't come up again until the fifth day. After solving five of Twilight Town's "Seven Wonders," and missing the opportunity of solving the sixth after intervention from DiZ via Hayner, Roxas sets off in search of the solution to the seventh. The rumor had it that a girl would appear in the second floor window from time to time, looking sadly out at the world. When Roxas looked up, he saw the mysterious girl he had met several days earlier, Naminé, looking back at him. He was then transported up into the room, where he and the girl held a strange conversation. When he returned, he and Pence looked up at the window once more, only to see the curtain fluttering in the breeze.
On the sixth day, Roxas began to fade from the digital world and merge with Sora. He could not be seen by anyone in the world, and simply passed through them. Only Axel, a member of Organization XIII and Roxas's one-time best friend, could tell he was there. Attempting to bring him back to the Organization, Axel becomes saddened by Roxas' lack of memory and finally hardens and decides to take him by force. DiZ freezes time momentarily to give Roxas a chance to escape, telling him to run to the mansion. Although the Nobodies assault him all the way through town, Roxas makes it to the mansion gate with the help of the Keyblade. Using Sora's memories, he unlocked the gate of the manor with the Keyblade and ran inside.
Inside, he met with Naminé once more, and began to regain his memories. It was within the mansion that he remembered that he was once a member of Organization XIII, that had used him for his power over the Keyblade, and he had left them to find his past, and that Axel was the only one who ever cared for him. After destroying the digital version of DiZ's computer and defeating Axel in the basement, he met with the sleeping Sora. The two merged here, and Sora awakened in the real Mansion.
 The Real Mansion
As seen in Kingdom Hearts II Final Mix, Roxas was born in the clearing just outside of the mansion, adding even more to the mystery of the mansion. After he was born, Xemnas arrives and gives him his new name, and offered him a place in Organization XIII. Also, in Kingdom Hearts II Final Mix, an Absent Silhouette and later, a data replica of Vexen challenges Sora, Donald, and Goofy to a battle in the courtyard. This is an allusion to Kingdom Hearts: Chain of Memories, when Vexen was fought here.
DiZ, surprisingly, chose the mansion as his base of operations after the happenings of Castle Oblivion, in which to start his revenge against Xemnas. He set up a large computer system in the hidden basement of the manor, set up a corridor and large room in which to hold the memory pods that were taken from Castle Oblivion, and a room on the second floor for Naminé to stay in. Together, he, Naminé, and Riku worked to restore Sora's memories. DiZ also set up the technology for a digital world, in which he placed Roxas after Riku captured him. So, in essence, the entire virtual world that Roxas stayed in is within the mansion itself.
Roxas eventually explores the mansion and goes through a number of puzzles to reach the basement, one puzzle is forces Roxas to complete a strange symbol on the table in the library. The symbol contains in it the Organization logo, the Heartless Emblem, and the crown from Sora's necklace. When Roxas and Sora finally merged, DiZ, Riku, and Naminé fled, letting Sora discover what has been happening on his own. Sora, Donald, and Goofy woke up in the basement levels, and began their second journey from there.
The Mansion isn't visited again until the last visit to the town. Twilight Town lay deserted when they first arrive, and the party continues through the town to the mansion, where they find Hayner, Pence, and Olette unconscious outside. They say that they had come to the mysterious manor in search of a second Twilight Town, where Sora's Munny Pouch and crystal must have come from. They figured that Kairi, whom they had become friends with, had ended up in the second Twilight Town after being captured, as they had no knowledge of the other worlds.
After making their way down to the basement, Pence activated DiZ's computer, and Sora, Donald, Goofy, and Mickey, whom they had met outside, went into the digital Twilight Town and went through a portal that Axel had left behind, bound for The World That Never Was. This portal leads to Betwixt and Between which only appears after dueling Axel. If examined before, then it only says, "It looks like it leads somewhere, but the power of darkness prevents entry."
 See also