The Castle of the Two Sisters is the currently abandoned ruins, and once home to Celestia and Luna prior to the event of Nightmare Moon. It also became the spot for one of the Warp Pads connecting Equus to both Earth and Homeworld.
Layout
It is located deep within the Everfree Forest and was once the birthhome to the princesses of Equestria. Despite having evidently been abandoned after Nightmare Moon's defeat, the castle remains in surprisingly good condition. The books in the library, despite being over a thousand years old, all appear in a usable condition, the various traps and pitfalls work smoothly, and the Organ to the Outside is in good working order and remains in tune. Even a small trapdoor that delivers food to Luna and Celestia's personal reading room appears to somehow still have access to fresh produce. The first new room in the castle to be shown is the library, which houses an abundance of ancient books and a large stone Alicorn bust. Star-spiders have also taken up residence in the library. The castle is shown to be filled with numerous secret doors and traps.
The throne room had two prominent thrones, one for both sisters to rule together.
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 said to have formed from remnants of Nightmare Moon's magic.
History
While unclear how it was built, it was indeed the first home to princess Celestia and princess Luna. It remained as such until Luna turned into Nightmare Moon, and as such destroyed much of the castle. While she did defeat her, Celestia had to move away from the Everfree Forest and to Canterlot not long afterwards.
Season 1
Curious over the Gems, Twilight went to the old Castle ruins after Celestia made the suggestion. Applejack and Rainbow Dash too went to the castle, mainly as a daring exploration due to the Pony of Shadows legend. This became the place where Steven Universe and Lapis Lazuli enter Equestria for the first time as well.
Season 3
While not fully presented on-screen, the Castle of the Two Sisters was one of the spots where the Warp Pads were built and tested.
Season 6
According to the "Butterfly Wings" alternate timeline, the Castle of the Two Sisters became the main, and pristine home to Nightmare Moon. Twilight Sparkle and Spike ended up going there not long after the timeline had changed, hoping to find Celestia.
My Little Universe II
By the time My Little Universe II started, the Castle became apart of Little Homeworld, being the main area connecting Earth to Equus.