The entrance to the sanctum is sealed with an incredibly complex sealing formula, only able to be decoded thoroughly by those possessing the Mangekyō Sharingan. The code itself was also inscribed by the eye's very first wielder, Madara Uchiha, as at the time, he was the only one who possessed such a power. In time, Sayuri Uchiha would also successfully gain entrance to the sanctum upon using her own Mangekyō Sharingan to do so.