Castle McDuck is the ancestral castle of the McDucks.
The Castle was built in the 10th century. It has been the McDuck's main fortress ever since, though other castles (including one on Loch Eerie and one near the town of Oldwick) have housed the Clan over time. Located in the middle of Dismal Downs, the castle is filled with memorabilia of the McDucks' heroic deeds and is also surrounded by a graveyard where most McDucks were buried. The castle (and the clouds above it) is also the usual home of all the ghosts of the deceased McDucks.
The castle has an elaborate network of secret passages and underground chambers, which used to be connected to the dungeons but became the secret dwellings they are now when Sir Swamphole McDuck sealed the entrance to the dungeons, as the McDucks could no longer afford taking care of it. They were then used as a hiding place for various treasures, such as the famed Knights Templar's treasure.
In the 17th century, the superstitious McDucks were chased away from their ancestral castle by a monstrous ghost hound (in truth a hoax orchestrated by a rival clan, the Whiskervilles, to get their hands on the fertile McDuck land). Living McDucks were not seen again in the castle until the later 19th century, when Fergus McDuck and his daughter Matilda and Hortense settled back in their ancestral home at Scrooge's input. Nevertheless, in 1902, Scrooge, Hortense and Matilda left for Duckburg and Fergus died, leaving the ghosts and a young man called Scottie McTerrier to take care of the castle.
In 1948, Scrooge McDuck and his nephews Donald, Huey, Dewey and Louie Duck visited Castle McDuck to look for Sir Quackly McDuck's lost treasure. After many adventures, they discovered Scottie McTerrier was dead and the old man claiming to be him was a jewel thief called Diamond Dick, who was a distant relative of the McDucks. Following the arrest of the fake McTerrier, Scrooge McDuck asked Donald Duck to find a new caretaker for the castle; Donald eventually hired his own aunt, Matilda, on the condition that Donald would keep Scrooge away from the castle (as she had sworn never to speak to her brother again).