The Sanctuary (Ezekiel 41:1-4).
We have now reached the central focus of the temple complex, the sanctuary itself. This was divided into three parts, the porch or vestibule ('ulam), the holy place (the nave - hekal - from the Sumerian e.gal ‘great house') and the holiest of all (the holy of holies) (here depicted by ‘debir' - from a root meaning ‘back' or ‘rear'). Everything up to this point has indicated the increasing holiness, from the seven steps by which entry was first made into the complex, past the necessary guard rooms, to the eight steps to the foursquare inner court, to the (ten) steps into the sanctuary itself, this emphasis continues. As a priest Ezekiel is able to enter the holy place, but notice that even he cannot enter the holy of holies, even in vision. That is allowed to the supernatural visitor alone. It is most holy.
How different it would all be once the heavenly temple was seen as transferred to Heaven in Revelation. Then Yahweh will be seen by all, although it must be recognised that the actions of those in His presence will still convey the idea of His holiness and greatness (Revelation 4-5). And there it has clearly been made possible because of the sacrifice of the Lamb Himself (Jesus Christ - Revelation 5:6).
The general plan of the sanctuary is patterned on the tabernacle and is paralleled elsewhere, for example at Tell Tainat on the Orontes where there is a small shrine dated 9th century BC patterned similarly, and at Khorsabad and Hazor (late bronze age).