The Old Chateau is one of the most mysterious areas in the Sinnoh region. You'll pass it while facing Pokémon Brilliant Diamond and Pearl's second Gym Leader, Gardenia, but you won't be able to enter it at first. Here's how to get inside the Old Chateau and a rundown of what you can expect to find inside this haunted mansion.
First, you'll need to acquire the TM 93 Cut, which allows you to cut down small trees in your path. To get this skill, you need to do two tasks: defeat Gardenia in her gym in Evisopolis, then approach the building above the PokéCenter in Evisopolis. A mysterious stranger will appear that you may or may not recognize and will give you the TM, allowing you to explore new areas.
You will find the ancient castle in the Evona Forest, just outside the town of Evonia. If you are inside the city, exit Route 205 and follow the trail into the forest. Once in the forest, you should see two small trees blocking a hole in a fence. Use the cut to eliminate them and advance through the newly formed entrance. You will then arrive at the Old Chateau.
The Old Chateau is haunted and as such is full of Ghost-type Pokémon; if you are looking for a Gastly, this is where you will want to go. Additionally, you can find some pretty unique items here, like Old Gateau, a disposable item that can cure any state ailment a Pokémon suffers from. The Old Chateau is also Rotom's home, but you won't be able to meet him until you've finished the game and unlocked the National Pokédex.