What Is a Reshus HaYachid
The Shulchan Aruch (O.C. 345:2) defines a reshus hayachid as an area that is enclosed by walls [mechitzos] that are at least ten tefachim high and encompass an area that is at the minimum four tefachim by four tefachim. These mechitzos can also encircle an area that is ten tefachim deep or an elevated area that is ten tefachim high.
Me’d’oraysa, if the mechitzos only enclose an area on three sides the area is still classified as a reshus hayachid (Tur and Shulchan Aruch, O.C. 363:1). At the minimum, each of the three sides must be omed merubeh al haparutz [that is, more than 50 percent of the length of each side must actually consist of a wall] for it to be considered whole for halachic purposes (ibid., 362:9-10, 363:1).
However, me’d’rabbanan, until the area is entirely enclosed it is classified as a karmelis. Therefore, in order that the area be reclassified, me’d’rabbanan, as a reshus hayachid we are required to rectify the fourth side of the enclosure ― and the pirtzos in the three sides ― with, at the minimum, a tzuras hapesach. Only then would it be permissible to carry therein.
 Once the walls are omed merubeh al haparutz on three sides, nearly all poskim maintain that the multitudes [rabbim] do not negate the enclosure, lo asu rabbim u’mevatlei mechitzta (Chacham Tzvi, siman 5, 37; Knesset Yechezkal, siman 2:3; Mayim Rabim, siman 34-36; Tosfos Shabbos, siman 363; Noda B’Yehudah, O.C. Mahadura Tinyana, 42 and Teshuvah M’Ahavah, siman 112; She’eilas Yaavetz, siman 7 and Mor U’Ketziyah, siman 363; Shulchan Aruch HaRav, O.C. 363:42, 364:4 and Kuntres Achron, O.C. 345:2; Bais Ephraim, O.C. 26; Keren Oreh, Eruvin 7a; Michtam L’David, siman 1; Chasam Sofer, O.C. 89; HaEleph Lecha Shlomo, siman 181; Aishel Avraham, siman 345; Chai Adam, klal 71:15; Chesed L’Avraham, siman 39; Maharham Shick, O.C. 171, 181; Maharia HaLevi, siman 94; Bais Shlomo, siman 43, 51; Tzemach Tzedek, Shabbos 100a and Eruvin, the end of Perek 5; Nefesh Chayah, siman 25; Avnei Nezer, O.C. 273:16, 279:2, 289:2; Aruch HaShulchan, O.C. 364:1; Maharsham, 3:188, 9:18; Yeshuos Malko, siman 21; Harei B’samim, 5:73; Imrei Yosher, siman 102 and Minchas Pitim, siman 364; Kaf HaChaim, O.C. 364:12; Divrei Malkiel, 3:10, 14; Rav Chaim Berlin in Tikkun Shabbos Odessa, p. 28 and in Nishmas Chaim, siman 29; Achiezer, 4:8; Even Yikrah, siman 58, and Chazon Ish, O.C. 74:10, 107:4; see also The Overwhelming Majority of Rishonim Maintain Lo Asu Rabbim U’Mevatlei Mechitzta).
It should be noted that had the Mishnah Berurah who paskens asu rabbim u’mevatlei mechitzta (Bi’ur Halachah 364:2) seen the Bais Ephraim (O.C. 26) in conjunction with the Mishkenos Yaakov (O.C. 120-122), there is a possibility he would have agreed with the Bais Ephraim that lo asu rabbim u’mevatlei mechitzta (see Part 1: According to the Mishnah Berurah, May a Baal Nefesh Carry in an Eruv of Tzuras HaPesachim?). See Bi’ur Halachah (208:9, s.v. Eino M’Vorech), where he states that he does not have the sefer Bais Ephraim (see also Toldos Shmuel, 3:81:7, 3:86:8; Bais Av, 2:5:2, and Even Yisroel, 8:36).
 The pirtzos in the mechitzos which are usually ten amos wide ― such as the streets running through the mechitzos ― would require at the minimum a tzuras hapesach; merely a post [lechi] would not suffice (Shulchan Aruch, O.C. 363:26). It is important to note that the tzuras hapesach can be utilized, me’d’rabbanan, to encircle a smaller part of the area enclosed by the mechitzos instead of closing the fourth side of the mechitzos themselves since the tzuras hapesach is being erected in a reshus hayachid d’oraysa.