Castle of Friendship

The Castle of Friendshipis a crystal castle made at some point during the Era of Two Sisters and was the residence of during the later days of the Era of Twilight.

History
After Twilight ascended to the throne and became the Queen of Equestria, the castle was either turned into a dormitory for students at the School of Friendship, or became a tourist attraction.

Architecture
The castle is fairly thin by castle standards, with a conical baseline instead the typical base of a castle. It is made of blue crystals that jet from the ground like branches of a tree with several jagged and sharp twist and turns. The living quarters of the castle are a more distinct shade of purple with small towers and arches. The roofs and balcony of the castle are yellow tiled ones, slightly resembling golden crowns. The top of the castle is marked by a striped, yellow-orange roof with three sharp indents and a light blue star.

In spite of its majesty, the castle was met with disapproval by several Equestrians The Pony described it as being an ugly site and Twilight herself found it hard to appreciate the castle after her homely and modest library was destroyed by Tirek.

IDW comics
To be written

The castle was often used as a backdrop and meeting area in the comics, rarely explored.

Twilight was in the castle when wrote to her.

OEL manga
The castle served as the initial setting for A Day in the Life of Equestria volume 1 chapter 1.

Twilight Sparkle was arranging books when came over and eventually entered her  to an alternate universe.

My Little Pony: Magic Princess
In the mobile game, the castle replaces the Golden Oak Library as a residential building in Ponyville after the Tirek quest is completed. It is home to several different characters, some of whom are variations of each other:

Young Twilight Sparkle
 * (when in Ponyville)
 * (when in Ponyville)
 * Spike
 * Twilight Sparkle
 * Spike
 * Twilight Sparkle
 * Twilight Sparkle