Middos, Chapter 1, Mishnah 3. Until now the chapter has been occupied with the guarding of the Beit Hamikdash and the rounds of the officer of the Har Habayit. Now the mishnah turns from the watchmen to the walls themselves, and takes us on a tour of the gates: how many there were, where each one stood, and what each one was used for.
Some orientation before we begin. On Har Hamoriah there was a square measuring five hundred amah by five hundred amah, and that square was Har Habayit, upon which the Beit Hamikdash stood. It was surrounded by a wall on each of its four sides, and no wall was left without an opening: the southern wall held two gates, while the western, northern and eastern walls each held one.
Five Gates
The mishnah begins: "Chamishah shearim hayu leHar Habayit", the Har Habayit had five gates.
Chuldah: the pair in the south
First the mishnah lists "shenei shaarei Chuldah", two gates called Chuldah, which stood "min hadarom", in the southern wall, and both of them were "meshameshin knisah vitziah", in use for coming in and for going out. Why should one side require a pair of openings when the others managed with a single one? Because the volume of traffic here was greater than anywhere else. The public approached Har Habayit from the south and returned home along that same road, and one doorway could not absorb crowds pressing in and out at once.
The name Chuldah recalls Chuldah haNeviah. According to the tradition she would sit between these two gates and teach the people during the days of the First Beit Hamikdash.
Kiponus in the west
"Kiponus min hamaarav, meshamesh knisah vitziah", the Kiponus gate in the west also served for entering and leaving. Kiponus is a Greek word, and it carries the sense of tending or working a garden. From the name we can gather that there was a garden, a rose garden, in Yerushalayim just beside the western side of the Har Habayit.
Tadi in the north
Next comes "Tadi min hatzafon", the Tadi gate in the northern wall, of which the mishnah says "lo hayah meshamesh klum", that it filled no role whatsoever. The public never entered through it and never departed through it. It was reserved for a single circumstance: should a kohen contract tumah, he was obliged to remove himself from the Beit Hamikdash without delay, and to do so without becoming a spectacle. Through this opening he could withdraw at once and unnoticed. Hence the name Tadi, which conveys privacy and discretion.
The eastern gate
Finally, "shaar hamizrachi", the gate in the east, and concerning it the mishnah records "alav Shushan Habirah tzurah": a representation of Shushan, the royal seat of Persia, was worked into it. Here is the history behind that image. When the first Beit Hamikdash lay in ruins and Israel was carried off into galut, they made their home in Persia. Later, when the Persian king authorized their return to Yerushalayim to rebuild, he did so with a stipulation attached: a likeness of Shushan was to be mounted over that gate, keeping the dread of Persian rule constantly in view so that no thought of revolt would take root.
The Route of the Parah Adumah
The mishnah continues: "shebo kohen gadol hasoref et haparah, uparah vechol mesaadeha yotzim leHar Hamishchah". Through this eastern gate the kohen gadol who burned the parah adumah, the cow itself, and all those who assisted him in the service, went out to Har Hamishchah, the Mount of Olives, which lies to the east of the Beit Hamikdash. The ashes of the parah adumah were mixed with spring water, and that mixture was used to purify.
Note that the mishnah here follows the opinion of Rabbi Meir, who holds that the burning of the parah adumah must be performed by the kohen gadol. The chachamim maintain that any kohen may carry it out.
So the eastern gate had a second function beyond serving as an ordinary entrance and exit. It gave the procession bringing out the parah adumah a direct route from the Har Habayit to the Mount of Olives, the place where the cow was ultimately burned.