1) THE WALLS OF A PIT
OPINIONS: According to Rebbi Yochanan, the Mishnah teaches that the height of the walls above and surrounding a pit join with the height of the pit itself to make a ten Tefachim height (so that the pit has a status of Reshus ha'Yachid).
Does the thickness of the walls join with the internal diameter of the pit to make a combined width of four Tefachim so that the pit will be a Reshus ha'Yachid if the internal diameter of the pit itself is less than four Tefachim?
(a) RASHI here (DH Mitztarfin) says that the walls join to make a height of ten Tefachim, so that if someone took an object from within the pit and placed it outside the pit, he would be Chayav for Hotza'ah. This implies that if someone would take an object from atop the walls of the pit, he would not be Chayav. It seems, then, that Rashi maintains that the thickness of the walls does not join the width of the pit.
However, RASHI in Eruvin (78a, DH Mitztarefes) says that just like the height of the walls joins to make ten Tefachim, so, too, the width joins to make four Tefachim.
Perhaps Rashi learns like TOSFOS (DH Mesayei'a) in our Sugya, who says that a pit joins with its walls to make a width of four Tefachim only if the wall itself is ten Tefachim high without the pit.
(b) The RITVA says that the walls join to make a width of four Tefachim no matter how tall the walls are. (See also Rosh in Eruvin.)
(c) The RAMBAM says that the walls of a pit do not join at all to make a width of four Tefachim. (See RAV CHAIM HA'LEVI, Hilchos Shabbos 14, who explains that the Rambam accepts the Yerushalmi's ruling that the walls join only if they are wider than the diameter of the pit. In such a case, the width of walls join to make the area on top into a Reshus ha'Yachid, because it is a usable area. The area between the walls themselves, though, is not considered to be Reshus ha'Yachid.)

99b----------------------------------------99b

2) THE AIRSPACE ABOVE A PILLAR IN "RESHUS HA'RABIM"
OPINIONS: The Gemara asks what the Halachah is in the following case. A person throws an object from Reshus ha'Rabim to the top of a pillar which is ten Tefachim high and four Tefachim wide (and thus has the status of a Reshus ha'Yachid). However, in order to land on the surface of the pillar, the object must travel through a Makom Petur. Is the person Chayav for such an act of Hotza'ah or not?
In the Gemara's case, at which point does the object travel through a Makom Petur?
(a) RASHI explains that the object travels through a Makom Petur when it goes above ten Tefachim in Reshus ha'Rabim before it reaches the top of the pillar. The airspace higher than ten Tefachim above Reshus ha'Rabim is a Makom Petur. The airspace above of the pillar itself, though, is considered Reshus ha'Yachid until the sky.
(b) The RASHBA and RITVA explain that the top of the pillar itself is a Makom Petur. They explain that a pillar is a weaker type of Reshus ha'Yachid than other types because it does not have Mechitzos around its surface to make it distinguishable as a Reshus ha'Yachid.
RABEINU CHAIM HA'LEVI (Hilchos Sukah) points out that with regard to a Sukah (4b), Rashi explains that "Mechitzos she'Einan Nikaros" (walls that are not recognizable; i.e., they exist only theoretically, as a result of a Halachic concept such as "Gud Achis") are not sufficient as walls for a Sukah. A Sukah needs actual walls and not just theoretical ones. The Ritva and Rashba perhaps maintain that even with regard to the laws of domains on Shabbos, theoretical walls are not sufficient, and Gud Achis does not work to make the top surface of the pillar considered surrounded by four walls. Although the thickness of the pillar is four Tefachim, only the top of the pillar itself is considered a Reshus ha'Yachid and not the air above it, unless there are walls which can be viewed to extend up until the sky so that the area between them is considered Reshus ha'Rabim.
3) AN OPEN VALLEY
QUESTION: RASHI (DH Mukaf l'Karmelis) implies that a Karpaf (open valley) that is larger than Beis Se'asayim becomes a Reshus ha'Yachid when it is fenced-in for the purpose of residential occupancy. If the Karpaf is smaller than Beis Se'asayim, it becomes a Reshus ha'Yachid when it is fenced-in even not for residential purposes. Rashi's words imply that a Karpaf that is larger than Beis Se'asayim and that was not fenced-in for residential purposes is considered a Karmelis. Rashi earlier (7a, 80a) reiterates his opinion that it is a Karmelis. However, this seems to contradict the Gemara earlier (7a), which says that such a Karpaf has the status of a Reshus ha'Yachid with the stringencies of a Karmelis. (GILYON HA'SHAS)
ANSWER: The Acharonim (see CHASAM SOFER) explain that Rashi (7a, DH Kegon) emphasizes that a Karpaf that was not fenced-in for residential purposes is considered a Reshus ha'Yachid only when it did not have any houses in it before the wall was erected around it. These words imply that a house was built in the Karpaf after the wall was erected. Perhaps Rashi maintains that a fenced-in Karpaf is considered a Reshus ha'Yachid only when a house is later built inside the walls. If, however, no house is ever built inside the walls, the Karpaf remains a full-fledged Karmelis.