Much like the original Castle McDuck, this imposing family home in Dismal Downs is the haunted ancestral home of the Clan McDuck, replete with secret passages of all kinds (courtesy of Sir Swamphole), which, in this case, house a demonic hound alongside the treasure of the Knights Templar. The Castle also includes a large dining room and a library, alongside hallways lined with portraits of nearly all McDucks.
The Castle was destroyed in some unknown event around the turn of the 20th century, while it was still inhabited by Fergus McDuck and his wife; to give them back their home, their son Scrooge McDuck acquired druidic rune-stones and attempted to manipulate them to rebuild the Castle. He more or less succeeded, but his inexperience with sorcery resulted in a few mystical side-effects; from then on, the inhabitants of the house (Fergus and Downy) would become immortal, and the mists surrounding the castle would only lift once every five years to allow safe passage.
Behind the scenes
Castle McDuck appears in the 2018 episode of DuckTales 2017, The Secret(s) of Castle McDuck — whose release, interestingly enough, is two months late of the original Castle's debut's 70th anniversary.