1. Rav Nachman: If there is a wall between two courtyards, the top of the wall belongs to the courtyard that can use it more easily.
2. The Gemara explains how to make the top of a wall (that is more than ten Tefachim high) usable.
3. Items stuck loosely into the ground may be taken out of the ground on Shabbos.
4. One may even stick a container into the ground to lessen the height from the floor of the yard to the top of the wall.
5. Anything that may be used on Shabbos may not be used to lessen the height of the top of the wall.
A BIT MORE
1. For example, in a case where the top of the wall is ten Tefachim above the ground for the lower courtyard but on the same level with the ground of the higher courtyard, the top of the wall is considered part of the higher courtyard.
2. If a person puts a four-Tefach-wide mound of dirt alongside the wall, thus causing the top of the wall to be less than ten Tefachim from the top of the mound of dirt, the residents of the courtyard may use the entire area atop the wall.
3. They may be taken out of the ground as long as they are partially sticking out of the ground.
4. Such a method may be used only if the container is placed in the ground in such a way that it would be necessary to use a shovel to dig the container out of the ground.
5. Since such an object may be used on Shabbos, it might be taken away on Shabbos and thus it is not considered part of the ground.