The Sanctum consistently has a circular window with four swooping lines; this design has stayed with the building despite the window's destruction on many occasions. The design of the window is actually the Seal of the Vishanti; it protects the Sanctum from most supernatural invaders. It is also called the "Window of the Worlds,".
After constructing the house, Doctor Strange cast a permanent, intricate spell of mystical force to protect it. Certain members of the New Avengers appear to acknowledge this. Chemistro, a super-villain member of the Hood's army, though possessing no such power to directly break that of the Vishanti's, was able to change the chemical composition of the wood that held the seal to break it. In one story Baron Mordo was able to transport the house to another dimension. During the World War Hulk, the Sanctum was partially invaded by the forces of the alien Warbound, its defensive enchantments and illusions shattered by Hiroim. After the use of unacceptable dark magics in the fight against the Hulk, the Sanctum is invaded by the Hood's army as mentioned before. The fight devastates the Sanctum, though the villains are defeated. Doctor Strange is forced to retreat when the battle allows the government-sanctioned Mighty Avengers to take over the Sanctum. Brother Voodoo is called in to neutralize the remnants of the defensive magics. On at least one occasion, Doctor Strange has destroyed the defenses of the Sanctum to avoid their exploitation by a foe.
Protection- The Seal of the Vishanti is a Magical ward used to protect auspicious locations from attack.
Impervious Protection 21
Marvel Database- Sanctum Sanctorum article- http://marvel.wikia.com/wiki/Sanctum_Sanctorum
Wikipedia Sanctum Sanctorum article- https://en.wikipedia.org/wiki/Sanctum_Sanctorum