It is a large building that appears to be located in the vicinity of the castle. It is guarded on the outside and within.
 Starswirl the Bearded wing
A section of the Archives that is locked off normally, but Twilight Sparkle and her guests are freely allowed to browse through without supervision.
The room has a number of shelves stacked with ancient scrolls, one of which contained a spell enabling Twilight to travel back in time for a few moments before returning automatically. The center of a room features a large hourglass that reaches approximately halfway up to the room's glass dome roof. The hourglass was bottom heavy with sand when last seen and sand was continuing to cascade down.
Curiously, in spite of being locked on the inside of the building, there is a large window that shows the room is in fact on the ground floor, thus being easily accessible to any pony willing to break the glass in order to enter illegally.
Twilight appears to have VIP status here because in spite of being caught in suspicious circumstances by a patrolling guard, he magically unlocks the wing without even being asked.
For whatever reason, Princess Celestia was seen visiting the wing for no apparent reason.