Middot, Chapter Four, Mishnah Two
1) The great gate had two small doors, one to the north and one to the south.
a) By the one to the south no one ever went in, and concerning it was stated explicitly be Ezekiel, as it says, And the Lord said to me: this gate shall be shut, it shall not be opened, neither shall any man enter in by it, for the Lord God of Israel has entered in by it; therefore it shall be shut (Ezekiel 44:2).
2) He [the priest] took the key and opened the [northern] door and went in to the cell, and from the cell he went into the Hekhal.
3) Rabbi Judah says: he used to walk along in the thickness of the wall until he came to the space between the two gates.
a) He would open the outer doors from within and the inner doors from without.
Section one: The great gate of the Hekhal had two small doors, one to the north (to the right when facing the Hekhal) and one to the south. However, the southern door was never used, due to a direct order by God.
Section two: When the priest wanted to open the great gate, he would take the keys to the gates, go into the cell, which was a chamber next to the gate, and then go into the Hekhal and open from the inside.
Section three: Rabbi Judah says that the priest would not enter the cell but would rather walk along inside the wall which was six cubits thick. He would then open the outer doors from within, turn around and open the inner doors from without.