Gemara

6b (Beraisa #1) Question: How can we Me'arev (permit carrying in) a Reshus ha'Rabim (with walls on two sides?)

Answer #1: One makes Tzuras ha'Pesach [across Reshus ha'Rabim] on one side, and Lechi or Korah on the other side.

Answer #2 (Chananyah): Beis Shamai requires a door on each side, and to lock it when going in or out;

Beis Hillel require a door on one side, and a Lechi or Korah on the other side.

Question: May one really be Me'arev Reshus ha'Rabim like this [according to Chachamim]?!

(Beraisa - R. Yehudah): If one owns two houses on opposite sides of Reshus ha'Rabim, he puts a Lechi (vertical post) on each side or a Korah (horizontal beam) on each side, and he may carry in the middle.

Chachamim: We cannot be Me'arev Reshus ha'Rabim like this!

Suggestion: Perhaps Chachamim say that we cannot Me'arev like this, but we can be Me'arev with a door [on one side, even if it is not locked, like Beis Hillel]!

Rejection (Rabah bar bar Chanah): Had they not locked the doors of Yerushalayim at night, one who carried in it would have been liable for carrying in Reshus ha'Rabim;

(Ula): If they didn't lock the doors of the Mavo'os Mefulashim of Mechuza at night, one would have been liable there for Reshus ha'Rabim.

Answer (Rav Yehudah): The Beraisa asks, how are we Me'arev Mavo'os Mefulashim? One makes Tzuras ha'Pesach on one side, and Lechi or (some texts - and) Korah on the other side...

(Rav): The Halachah follows the first Tana.

(Shmuel): The Halachah follows Chananyah.

Question: According to Beis Hillel according to Chananyah, must one lock the door?

Answer (Rav Yehudah and Rav Masnah, citing Shmuel): One need not lock it.

Question: Must one lock the doors?

Answer #1 (Rav Anan): The [doors of the] Mavo'os Mefulashim are half-covered in dirt (they cannot be closed), yet Shmuel walks through and does not comment!

Rejection (Rav Kahana): Those doors were only slightly open.

Answer #2: Rav Nachman commanded to clear the dirt away from the doors [to enable closing them].

Suggestion: Rav Nachman holds that the doors must be closed.

Rejection: No, he requires only that it is possible to close them.

22a (Mishnah - R. Yehudah): (Chachamim enacted Pasei Bira'os. If four L-shaped corner-posts [of a certain size] surround a well, one may take from the well and carry inside the posts.) If Reshus ha'Rabim passes through the posts, it must be diverted.

Chachamim say, there is no need for this.

(R. Yochanan and R. Elazar): This (Chachamim's opinion) teaches the strength of Mechitzos. (Traffic is not Mevatel them!)

Question: R. Yochanan taught that had they not locked the doors of Yerushalayim at night, it would have been Reshus ha'Rabim. This is unlike Chachamim!

Answer: He says that Chachamim's opinion teaches the strength of Mechitzos, but he himself holds like R. Yehudah.

Rishonim

Rif and Rosh (Eruvin 2a and 1:8): Tzuras ha'Pesach suffices for Mavoyos Mefulashim to Reshus ha'Rabim. Reshus ha'Rabim itself requires locked doors on both sides, like R. Yochanan taught.

Rambam (Hilchos Shabbos 17:10): If there are two walls in Reshus ha'Rabim and people go between them, one puts a wall on each side, and inside is Reshus ha'Yachid. One need not lock the walls at night, as long as they could be locked.

Magid Mishneh: Regarding a Mavoy Mefulash, the Rif rules like Rav, who rules like the first Tana. The Rambam agrees, like he wrote at the beginning of this Perek. Regarding Reshus ha'Rabim, the Rif and Rambam require doors at both ends. The Rambam holds that the doors need not be locked, for Rabim are not Mevatel Mechitzos, like one opinion on 22b. He holds that our Sugya discussed Chananyah because it is relevant to us to be Machshir Reshus ha'Rabim. The gates of Neharde'a enclosed a proper Reshus ha'Rabim, and it sufficed that they could be locked. The Rif rules like R. Yochanan, that for Reshus ha'Rabim they must be locked. The Rashba rules like Chananyah regarding a Mavoy Mefulash, and he requires doors for Reshus ha'Rabim. Reshus ha'Rabim is a place where people may go at any time. If the doors are locked at night, this is not fulfilled. The Rambam's opinion is primary.

Poskim

Shulchan Aruch (OC 364:2): Reshus ha'Rabim is permitted only through doors. They must be locked at night. Some say that it suffices if it is possible to lock them.

Beis Yosef (DH u'Reshus): The Rambam rules that it suffices if the doors could be locked. He holds that since the Halachah does not follow Chananyah (we rules like Rav against Shmuel regarding Isurim - PF), the Gemara asked about his opinion because it is relevant to us regarding what is needed to be Machshir Reshus ha'Rabim. Rav Nachman said that it suffices that they could be locked. Therefore, for us this suffices to be Me'arev Reshus ha'Rabim. Since the Rif and Rosh agree, we rule like them.

Magen Avraham (2): Tosfos (6b DH veha'Amar, citing the Ri) and the Rashba say that it suffices that one door is locked at night, and there is a Lechi or Korah on the other side. Rashi requires two doors. A door is not a full Mechitzah, therefore a door is needed also on the fourth side. The Rif and Rosh connote like this.

Gra (DH v'Yesh): We find that the doors and gates of Neharde'a were not locked. Surely it was Reshus ha'Rabim, just like in Mechuza. (One would have been liable for carrying in Mechuza if not for the doors.) And all the more so according to his (the Rambam's) opinion that we do not need 600,000 to make Reshus ha'Rabim.

Damesek Eliezer: One might have thought that Neharde'a was not a real Reshus ha'Rabim, and therefore the doors did not need to be locked, like Chananyah, but for a Reshus ha'Rabim the doors must be locked. Therefore, the Gra says that surely it was like Mechuza. Also, the Rambam does not require 600,000 to make Reshus ha'Rabim, and even so it sufficed that the doors could be locked.

Mishnah Berurah (6): Through doors on both ends it becomes like a Chatzer of Rabim. Tzuras ha'Pesach does not suffice (even from both sides). Some say that it suffices to have a door on one side if it is locked at night. This is not Reshus ha'Rabim, for it stops people from going from gate to gate. It suffices to have a Lechi or Korah at the other side, like in general, when there are three full walls.

Mishnah Berurah (7): Since they were locked at night, it is unlike the encampment in the Midbar, which was always open.

Mishnah Berurah (8): "Some say that" connotes that the Mechaber holds like the first opinion. The Shulchan Aruch clearly requires doors for Reshus ha'Rabim. Our custom is to rely on Tzuras ha'Pesach, even though our streets are very wide and Mefulashim from gate to gate. Also, sometimes the king's path goes through the city. According to letter of the law, this is a full Reshus ha'Rabim. We must say that we rely on the opinion that Reshus ha'Rabim requires 600,000 going through every day, and this is not common nowadays. It is proper not to rely on this. See the Bi'ur Halachah.

Bi'ur Halachah (DH v'Hu): The Rif, Rosh, Semag and other Poskim rule like R. Yochanan. If the doors are not locked, and all the more so if there is merely Tzuras ha'Pesach, one is Chayav Chatas. Rabim are Mevatel the Mechitzah. The latter opinion is the Rambam, who rules like R. Elazar. Even Tzuras ha'Pesach exempts from Chatas. The Rabim are not Mevatel Mechitzos. The Rambam agrees that mid'Rabanan one needs a door.

Bi'ur Halachah (DH v'Achar): Why is the custom to rely on Tzuras ha'Pesach in Reshus ha'Rabim that is 16 Amos wide and open from one end to the other? We rely on the Rambam, who rules like R. Elazar, that Rabim are not Mevatel Mechitzos. Surely he holds that mid'Oraisa, Tzuras ha'Pesach suffices. Since only an Isur mid'Rabanan remains, we may rely on the opinion that Reshus ha'Rabim depends on 600,000, so Tzuras ha'Pesach suffices for Karmelis. This Heter is not clear. Most Poskim rule like R. Yochanan, and that we have Reshus ha'Rabim. One may not protest against the masses who are lenient, but a Ba'al Nefesh should be stringent for himself, for it pertains to a Chiyuv Chatas. Also Beis Meir says so.

Kaf ha'Chayim (12): Rabbeinu Zalman says that even though mid'Oraisa, Tzuras ha'Pesach is a full Mechitzah, mid'Rabanan we require doors.

Kaf ha'Chayim (14): Even if a wall surrounds the entire city, if the gates are aligned with each other and the doors are not locked at night, it is Reshus ha'Rabim. If the doors are locked at night, it is Reshus ha'Yachid, but an Eruv is needed. Without an Eruv, it is a Karmelis.

Kaf ha'Chayim (15,16): The Mechaber requires that the doors are locked. The Darchei Moshe follows the Magid Mishneh and Tur, who say that the Rambam's opinion is primary. The Mordechai (Reish Perek ha'Dar) says that R. Yochanan obligates locking the doors even nowadays that we do not have a proper Reshus ha'Rabim; Maharam holds that nowadays they need not be locked. Tosefes Shabbos says that similarly, he holds that it suffices to have Tzuras ha'Pesach on one side and Lechi or Korah on the other side. This is the custom, but one who fears Shamayim should be stringent. It seems that the Rema did not comment here because he retracted from what he wrote in Darchei Moshe.