Its Japanese name is the same as that of the Old Gateau, with the difference being that the Old Gateau is instead spelled as もりのヨウカン (with the second word in katakana instead of hiragana). According to Nob Ogasawara, he was able to keep the Old Chateau/Old Gateau pun in the English versions in part due to him being able to give more input in the localization process from Generation III onward.
The Old Chateau is located deep within Eterna Forest, near the Route 205 entry point from Eterna City. It cannot be reached without using Cut on an obstructive small tree. The manor is known to be haunted, scaring many citizens of Eterna City, especially Gardenia, who sometimes appears before it cowering in fear at the urban legends related to it.
Upon entering, the Old Chateau appears to be old and abandoned. The first room encountered is the foyer, which has two flights of stairs both leading to the second floor. Between the foyer's staircases is a door to the dining hall, where a long wooden table dominates the room. The kitchen area is found to the left side of the table. Just outside of the room stands a strange statue with an ominous aura that appears to be glaring.
On the second floor, left and right of the main room extends a library, where old books are crammed into bookshelves. There are also four bedrooms that can be accessed by traveling further up to the first floor. In one of the rooms there is a TV that is on and displaying a plain, white, flickering screen. The TV is inhabited by a wildRotom, which comes out exclusively at night to initiate a battle (in Pokémon Diamond and Pearl, this requires entering the Hall of Fame and obtaining the National Pokédex). In another room, there is a portrait with glowing red eyes that stare at and seem to follow the player, but only while the player is in that room.
Occasionally, the ghosts of a little girl and a butler appear in the rightmost bedroom on the second floor and in the dining hall, respectively. They cannot be interacted with and move by floating.
The little girl appears when the player enters the fourth room from the west, which is next to the bedroom where she randomly "appears". When this happens, she looks to the left, where the player is, then moves towards the exit, never being seen on the other side. This can only occur when the player enters this particular room because the game loads a different map depending on the room entered, and the little girl is only programmed to appear in the map of the fourth room.
The butler appears to be sitting at the other side of the table in the dining room. The player walks forward and he starts moving to the left—leaving the game window—as if going to the kitchen area, but never appearing there.
The little girl who appears in the bedroom
The butler who appears in the dining hall
In Pokémon Platinum, one of the books on the floor in the library is just barely readable. It says:
It's an old notebook. There's something scrawled in it...
"Som...hing so pecu...r shou... make off ...ith the mot..."
The rest of the pages are blank.
ふるびた ほんに かきなぐった あとが ある⋯⋯
あんな えたいのしれ⋯⋯ ⋯⋯に モ⋯⋯を もっていかれる とは⋯⋯
It most likely is meant to say: "Something so peculiar should make off with the motor."
The Old Chateau is located in Canalave City in the anime. Here people can make the Old Gateau using the instructions and ingredients found there. It is also run with an automated voice to guide the people visiting the mansion, which also served as a hotel.
In the episode Get Your Rotom Running!, it was abandoned for a while due to electricity problems from a recent thunderstorm. As a result, a Rotom took residence there for a while to play with the people coming in by possessing different appliances.
The Old Chateau appeared as the focus point of the episode The Old Chateau. Cheryl and her Chansey were drawn there while looking for a place to stay overnight while exploring the Eterna Forest. While inside, the two were tricked and eventually attacked by the ghosts of a butler and a little girl.
The Chateau's interior
The little girl
The Haunter illusion
In the manga
In the Pokémon Adventures manga
In Robust Rotom, Diamond and Pearl discover the Old Chateau while leading Platinum through the Eterna Forest. Platinum, unaware that the manor-like building had long been deserted, decides that she and her bodyguards stay overnight inside. During their stay, a globe of energy emerges around Diamond and Pearl, spooking them in the process. Diamond manages to lure the energy globe into a television in one of the rooms. Subsequently, the trio enjoy a peaceful night's sleep before leaving the next morning.
In Unplugging Rotom (Heat, Wash, Mow, Fan, Frost), Diamond and Pearl return to the Old Chateau to seek out the Pokémon they had seen before, as they were co-operating with the Sinnoh Pokémon Association to stop Charon from capturing the Pokémon that were detailed in his notebook. Charon had referred to the Old Chateau's plasma-like substance as Rotom. This time, Rotom emerges from the television and flees the chateau.