Keilim, Chapter 1, Mishnah 8. Our Mishnah stands in the middle of a list that ranks places one above another in kedushah, every rung higher than the one before it. The list has already arrived at those cities in Eretz Yisrael that are surrounded by a wall, and from here it travels inward: first Yerushalayim, then Har Habayit, and from there, courtyard by courtyard, into the Beit Hamikdash.
The layout of Har Habayit
To follow the Mishnah, it helps to picture the place. Har Habayit was a square measuring 500 by 500 amot, enclosed by walls. Inside that large square stood the courtyards of the Beit Hamikdash, which had walls of their own. Running around the outside of the courtyard walls was a strip ten amot wide called the cheil, and the cheil was marked off by a wooden lattice fence known as the soreg. (These measurements and structures are described at length in Masechet Middot.) East of the courtyard lay the women's courtyard, the Ezrat Nashim, a square of 135 by 135 amot.
The azarah itself was subdivided from east to west into bands of rising kedushah. Coming in from the eastern side, one first met the Ezrat Yisrael, a strip eleven amot deep. Directly past it lay a second strip of eleven amot, the Ezrat Hakohanim. Then came a span of thirty two amot in which the outer mizbeach stood, and from the mizbeach until the Heichal there were a further twenty two amot.
Yerushalayim over the other walled cities
Our Mishnah begins with Yerushalayim: "lifnim min hachomah", the space enclosed by the city's wall, is "mekudash meihem", of a higher sanctity than those cities. Both Yerushalayim and the other walled towns of the land carry the din of machaneh Yisrael, so what lifts Yerushalayim above them? The Mishnah supplies the reason: "she'ochlim sham kodashim kalim", for kodashim kalim are eaten there, "uma'aser sheini", and so is ma'aser sheini.
Korbanot fall into two categories. Kodshei kodashim, the offerings of the highest sanctity, may be eaten only by kohanim, and only within the courtyard of the Beit Hamikdash. Kodashim kalim, offerings of lesser sanctity, may be eaten even by those who are not kohanim, but their permitted area is all of Yerushalayim and nowhere beyond it. The same is true of ma'aser sheini, the second tithe separated in certain years of the cycle, which must be eaten in Yerushalayim. In the other walled cities neither kodashim kalim nor ma'aser sheini may be eaten. That is Yerushalayim's added holiness.
Har Habayit over the rest of Yerushalayim
The next rung: "Har Habayit mekudash mimenu". The mountain bears a sanctity above that of Yerushalayim at large, and the Mishnah states why: "she'ein zavim vezavot, nidot veyoldot nichnasin lesham". A man in a state of zivah, a woman in a state of zivah, a woman who is a niddah, a yoledet (a woman after giving birth): not one of them may set foot on the mountain, though the streets of the city remain open to them without restriction.
The cheil over Har Habayit
"Hacheil mekudash mimenu". The ten amah band running along the outside of the azarah walls carries a kedushah beyond that of the remainder of the mountain, "she'ein goyim ut'mei met nichnasim lesham": a non Jew, and a Jew who contracted tumah from a corpse, are turned back at the cheil, even though Har Habayit itself is open to both of them.
Two halachic notes underlie this. A person who became tamei through a corpse is not barred by the Torah from machaneh Leviyah; his exclusion from the cheil and everything within it is a takanah of the Chachamim. The bar on a non Jew is likewise of rabbinic origin. The Chachamim assigned a non Jewish man the din of a zav, and a non Jewish woman the din of a niddah, both on the rabbinic level. Following that logic, one would have expected them to be kept off Har Habayit altogether, just as the Mishnah rules for a zav and a zavah. The Chachamim chose to be lenient here, giving them instead the standing of one who is tamei met, so that they would not be denied the chance to draw near the Beit Hamikdash and offer their prayers. They may therefore advance as far as the cheil, and no further.
The Ezrat Nashim over the cheil
"Ezrat nashim mekudeshet mimenu", the women's courtyard is holier than the cheil. It too has the status of machaneh Leviyah, and it was enclosed by a wall separating it from the cheil. Its added stringency: "she'ein tevul yom nichnas lesham", a tevul yom may not enter, even though he is permitted in the cheil.
A tevul yom is a person who has already immersed in a mikveh to remove his tumah but is not yet fully tahor, because nightfall has not yet come. By Torah law a tevul yom may enter machaneh Leviyah, and it was a decree of the Chachamim that forbade him the Ezrat Nashim while still allowing him the remainder of Har Habayit.
The Mishnah adds: "ve'ein chayavin aleha chatat", one who entered while tamei is not liable to bring a chatat. A chatat is brought only for an unintentional transgression that would carry kareit had it been done deliberately. Since entering the Ezrat Nashim in a state of tumah is forbidden only rabbinically, there is no kareit involved, and therefore no chatat when it was done by mistake.
The Ezrat Yisrael over the Ezrat Nashim
"Ezrat Yisrael mekudeshet mimenah". The Israelites' courtyard, which already bears the status of machaneh Shechinah, ranks a level above the Ezrat Nashim, and the Mishnah explains: "she'ein mechusar kippurim nichnas lesham". A mechusar kippurim is kept out of it, while the Ezrat Nashim is open to him.
A mechusar kippurim is one who had a form of tumah that requires korbanot for its final purification: he immersed in a mikveh, nightfall passed, but he has not yet brought the offerings he owes. His tumah has been greatly weakened by the immersion and by the coming of night, yet the missing korbanot still bar him, and here the prohibition is by Torah law, not merely rabbinic.
The Ezrat Hakohanim over the Ezrat Yisrael
"Ezrat hakohanim mekudeshet mimenah". The kohanim's courtyard stands one level above the Ezrat Yisrael, "she'ein Yisrael nichnasim lesham ela bish'at tzorcheihem": a Yisrael who is not a kohen crosses into it only when the avodah gives him cause to, "lismichah, lishchitah velitnufah", to perform semichah (pressing his hands upon the animal's head), to slaughter his korban, or for tenufah, the waving of certain portions of certain korbanot. Without such a need, he remains outside.
This limitation as well was instituted by the Chachamim; the Torah sets no boundary at this point. Their ruling keeps a Yisrael out of the kohanim's courtyard, whereas the Ezrat Yisrael stands open to him whenever he wishes. Precisely that distinction places the Ezrat Hakohanim on a higher level of kedushah than the Ezrat Yisrael, one more step in the Mishnah's climb toward the Beit Hamikdash.