Much like unwelcome guests a maze of extradimensional vault doors protect some of the more dangerous mystic artifacts in Strange's collection. If placed inside a mundane building Strange’s Vaults would be twice the size the Sanctum.
The hall and walls of the Vaults move under their own power; the Vault Hall shifts and folds to bring the patron and desired vault together within.
Occult Artifacts in the Vault
Arch of Nihility disguised as a radio
Table-lamp composed of actual serpents
Third Eye of Horus (formerly)
Window with constant rain
Winking wardrobe with a gateway to a demon realm
Marvel Database- Sanctum Sanctorum article- http://marvel.wikia.com/wiki/Sanctum_Sanctorum
Wikipedia Sanctum Sanctorum article- https://en.wikipedia.org/wiki/Sanctum_Sanctorum