Almost nobody who visits Ise is allowed to see what the shrine is actually built to protect.
Two shrines, and what each guards
Ise Grand Shrine is really a pairing: the Naiku, or Inner Shrine, dedicated to the sun goddess Amaterasu, and the Geku, or Outer Shrine, dedicated to the harvest deity Toyouke. The Naiku is traditionally held to house the Yata no Kagami, the sacred mirror counted among the three Imperial Regalia of Japan, though no outsider, and reportedly not even the reigning emperor, ever views it directly; it is described in tradition as concealed within layered coverings inside the sanctuary.
Who may go where
Visitors, including most of the Japanese public, may approach only as far as an outer fence and gate; the innermost buildings are entered only by Shinto priests and, on particular occasions, members of the imperial family. A hereditary or imperially appointed high priestess role, the Saigu in earlier history and now a position typically held by a member of the imperial family, has overseen the shrine's most sacred observances.
A boundary that outlasts every rebuilding
The shrine's buildings are torn down and rebuilt every twenty years, but this boundary of restricted access has stayed constant across every reconstruction, making secrecy, not the timber itself, the site's most durable feature.