Gemara

6a (Beraisa): A fence that is 10 Tefachim tall and four wide is a full Reshus ha'Yachid.

96a (Mishnah): If two ledges jut out from houses on the same side of Reshus ha'Rabim, and one was Moshit (passed an object) from one to the other, he is liable.

99a (Rav): The wagons, underneath them, between them and to the sides of them were Reshus ha'Rabim.

101a (Rav Huna): If a small boat is four wide, until within three Tefachim of the bottom, one may carry on the whole boat.

Question (Rav Nachman): [You should permit even if it is four wide only at the top, due to Gud Achis Mechitzta! (It is as if the wall extends downward, from where it is four wide.)

(Beraisa - R. Yosi b'Rebbi Yehudah): If one inserted a pole in Reshus ha'Rabim with a basket (four by four) on top, and one threw something (from Reshus ha'Rabim) and it landed on the basket, he is liable.

Answer (Rav Yosef): Rav Yehudah taught that Chachamim [argue with R. Yosi b'Rebbi Yehudah, and] exempt!

Question (Abaye - Beraisa): If a pillar in Reshus ha'Rabim is 10 tall and four wide, but at the bottom it is less than four wide, if the narrow part has three, if one threw [from Reshus ha'Rabim] and it landed on top of it, he is liable.

This shows that we say Gud Achis!

Regarding the basket Chachamim do not say Gud Achis because goats can stick their heads through [the projected extension of the walls]. This does not apply to the pillar or the boat.

Question (Rav Acha brei d'Rav Acha): Fish can break through [the projected extension of the walls of the boat]!

Answer (Rav Ashi): We are concerned for penetration of goats, but not of fish. (They are covered by the water.)

Rishonim

Rambam (Hilchos Shabbos 14:1): Reshus ha'Yachid is a mound at least 10 Tefachim tall and four by four Tefachim wide, or a ditch at least 10 deep and four by four wide, or any place surrounded by four walls 10 tall enclosing at least four by four. Even if it is several Mil, if it was Hukaf l'Dirah (surrounded for the sake of residence), it is a full Reshus ha'Yachid.

Tosfos (99a DH Shtei): We must discuss ledges that are sealed below, so goats cannot go underneath, or there are walls on top. Then, we are not concerned for goats. The wagons were Reshus ha'Yachid, even though underneath was Reshus ha'Rabim, for from the bottom of the wagon (the part that held the Kerashim) until the top of the Kerashim was more than 10 Tefachim.

Tosfos (101a DH v'Yesh): Rashi explains that "the narrow part has three" means that it is three above the ground. This is difficult. If it is very thin, this is like a basket [on a pole]. Goats go through, so we do not say Gud Achis! Rather, the Ri explains that the narrow part is three wide. Since it is three, and above is four, Gud Achis applies.

Tosfos (101a DH v'Zarak): The Gemara thought that Mechitzos over water are just like other Mechitzos, and Rabanan do not argue with R. Yosi b'Rebbi Yehudah. A Mishnah (Eruvin 87b) requires a Mechitzah of 10 Tefachim around a balcony above water in order to draw water. We could not have asked that we should not need a Mechitzah, for we can say Gud Achis, like regarding the basket. R. Yosi says only that Gud Achis makes above is Reshus ha'Yachid, but not below. Alternatively, in Eruvin we require a Mechitzah to permit Tiltul (e.g. drawing water up to the balcony), but [liability for] throwing does not require a Mechitzah, just like a basket. The Gemara asked that just like R. Yehudah holds that Gud Achis makes Reshus ha'Yachid to be liable without a Mechitzah, we should use Gud Achis to permit Tiltul.

Rashi (101a DH Mishum): We are not concerned for goats passing under the slope of the pillar. It does not slope so much. They do not come close to scratch themselves on it.

Poskim

Shulchan Aruch (OC 345:2): Reshus ha'Yachid is a place surrounded by walls 10 Tefachim tall and it has at least four by four Tefachim, or a ditch at least 10 deep and four by four wide, or a mound 10 tall and four by four wide.

Magen Avraham (1): A sloped mound whose walls rise 10 Tefachim within four Amos is Reshus ha'Yachid. If the walls rise 10 Tefachim within five Amos, and it is easy to use, it is Reshus ha'Rabim if the mound is in Reshus ha'Rabim.

Mishnah Berurah (5): If it slopes 10 Tefachim over [a horizontal distance of] four Amos, it is as if it is vertical. It is Reshus ha'Yachid where it is 10 tall.

Kaf ha'Chayim (12): The mound [with a steep slope] is considered Reshus ha'Yachid because we apply Gud Achis on every side, so the top is surrounded by Mechitzos. This is a tradition from Moshe from Sinai.

Magen Avraham (1): If a basket four Tefachim wide is on top of a beam 10 Tefachim tall, we do not say Gud Achis, for goats pass through. If the basket itself is 10 by four, also what is under it is Reshus ha'Yachid. If a beam is 10 by four, but at its base it is not four wide, even without goats the bottom is not Reshus ha'Yachid. Only on top it is Reshus ha'Yachid (Tosfos 99a).

Gra (on Sa'if 1): There is a printing mistake in the Magen Avraham. The basket becomes Reshus ha'Yachid due to Gud Achis, but underneath it is not Reshus ha'Yachid. If a beam is 10 by four, underneath is Reshus ha'Yachid only it is at least three Tefachim wide below, like Tosfos 101a.

Machatzis ha'Shekel: There is a printing mistake in the Magen Avraham. Tosfos (99a) says that the wagons were Reshus ha'Yachid, for from their bottoms until the top of the Kerashim was 10. Even so, underneath was Reshus ha'Rabim. Seemingly, one answer in Tosfos (101a DH Zarak) says just like the Magen Avraham, that underneath the basket is Reshus ha'Yachid if the basket has a Mechitzah 10 tall. However, this is wrong. This was only in the Hava Amina that Rabanan agree with R. Yosi. Really, they argue. Since goats can pass under the basket, it is not Reshus ha'Rabim even if the basket is 10 tall. This is not a problem over water, for we are not concerned for penetration of fish. Also Tosefes Shabbos corrected the text of the Magen Avraham to say that under the basket is Reshus ha'Rabim.

Damesek Eliezer: The Magen Avraham says that under the basket is Reshus ha'Yachid when the basket itself is 10 tall and four wide, but not if it is four wide only at the top. He teaches that a under a pillar is not Reshus ha'Yachid when it is four wide only above. One is liable only for throwing onto it. In this case, one is liable for throwing onto the pillar, and exempt for throwing on the basket. If it is four wide for 10 Tefachim, and below it is less than four, in both cases above is Reshus ha'Yachid, and below is not. The Magen Avraham says that in this case underneath is Reshus ha'Yachid. The Gra said that this is a printing mistake, for under the wagons was Reshus ha'Rabim. Rather, it should say Reshus ha'Rabim.

Objection (R. Akiva Eiger): Tosfos clearly says that under the basket is not Reshus ha'Rabim! (He says that one is liable for throwing there from Reshus ha'Rabim. I.e. mid'Oraisa it is Reshus ha'Yachid, but mid'Rabanan Tiltul is forbidden - PF.) I found that Acharonim pointed this out.

Gra (DH Mekom): The Gemara says so [that a place surrounded by walls is Reshus ha'Yachid] in many places, primarily in 6a. "Geder" is unlike Rashi.

Damesek Eliezer: "Geder" is a place surrounded on four sides. The Rambam explains like this. Rashi said that it is a wall 10 by four. This is not called a Geder, rather, a mound [or] pillar. Be'er ha'Golah says that the Shulchan Aruch's source for "a place surrounded by walls" is in the Beraisa, i.e. "v'Chen Geder."

Kaf ha'Chayim (7): We permit Parutz k'Omed [half of the width of the Mechitzah is missing. Even so, it is considered a Mechitzah.] See 362:9.

Kaf ha'Chayim (8): The Levush says that a tradition from Moshe from Sinai requires four Tefachim.

Kaf ha'Chayim (10): If it was Hukaf l'Dirah, there is no upper limit to the size. If it was not Hukaf l'Dirah, and it is more than Beis Sa'atayim, mid'Oraisa it is Reshus ha'Yachid, and mid'Rabanan it is Karmelis.