Castle of the Two Sisters

The Castle of the Royal Pony Sisters, alternatively called the Castle of the Two Sisters, is the castle that Princess Celestia and Princess Luna used to live in a thousand years prior to the beginning of the series. It's located deep within the Everfree Forest, and now stands abandoned and in ruins after much of it was destroyed by Nightmare Moon.

Season one
The castle is first introduced in the second part of the season one premiere, mentioned in the book The Elements of Harmony: A Reference Guide. It specifies that the Elements of Harmony's last known location is in the ancient castle of the royal pony sisters, and that the castle itself is located in what is now known as the Everfree Forest. Twilight Sparkle and her friends travel to the castle in order to retrieve the Elements and defeat Nightmare Moon.

In this episode, only two rooms in the castle are featured: the foyer, where the Elements of Harmony are found, and the throne room, where Twilight and her friends defeat Nightmare Moon.

Season four
In the season four premiere, Princess Twilight Sparkle - Part 1, Twilight views a flashback in which the castle is still in good condition. Unlike in Friendship is Magic, part 2, the throne room is shown to have two thrones instead of one. Within the flashback, Twilight witnesses Princess Luna destroy parts of the throne room and transform into Nightmare Moon.

In Princess Twilight Sparkle - Part 2, Nightmare Moon attacks Princess Celestia, destroying more of the throne room and much of the castle's surrounding architecture in the process. The Elements of Harmony are also shown, when Princess Celestia raises them up from a chamber underneath the throne room floor.

The castle and its inner workings are featured in greater detail in Castle Mane-ia when Twilight and her friends all find their way to the old castle for varying reasons. The first new room in the castle to be shown is the library, which houses an abundance of ancient books and a large stone unicorn bust. Star-spiders have also taken up residence in the library.

The castle is shown to be filled with numerous secret doors and traps, some of which Twilight's friends inadvertently set off and get themselves trapped in and most of which are controlled by a large pipe organ in the castle basement. Other rooms and areas in the castle include a reading room behind a library bookcase where Twilight finds Celestia and Luna's diary, the "Hall of Hooves"—a hallway lined with torch wall-mounts shaped like pony legs—a corridor lined with suits of pony armor, and a basement chamber that houses the aforementioned "Organ to the Outside". External shots of the castle show dragon statues on its ledges.

In the time following the castle's abandonment, it has also become the subject of Equestrian urban legends, such as the "Pony of Shadows", an ethereal being formed from remnants of Nightmare Moon's magic.