1. The Gemara discusses how one may decrease an area that is more than 70 by 70 Amos that was not made for dwelling.
2. The Gemara discusses whether building another wall within the original wall is effective to make an area considered walled for dwelling.
3. Everyone agrees that building a wall on top of a pre-existing wall is not a valid form of Kinyan for acquiring an ownerless area.
4. There is a dispute about whether building a wall on top of a pre-existing wall gives the area the status of an area walled for residential purposes on Shabbos.
5. Walls made for the purpose of dwelling do not function as such if they happen to surround an orchard as well.
A BIT MORE
1. Rava: In order to make the area less than 70 by 70 Amos, one may build a platform that is at least ten Tefachim tall and four Tefachim wide. Rabah: The platform is effective even if it is more than three Tefachim wide.
2. Rava: The new wall is effective if it is at least four Tefachim away from the old wall. Rabah: The new wall is effective even when it is more than three Tefachim away from the old wall.
3. Ownerless land is acquired by an act that shows one is taking possession of the land, such as by plowing, erecting a fence, etc. Building a wall on top of a pre-existing wall around the area is not a valid form of acquisition.
4. Rav Chisda: The addition of the upper part of the wall gives the area the status of an area walled for residential purposes. Rav Sheshes: It does not.
5. If wall built around a house (i.e. walls around an estate) are next to an orchard, and the house is within the walls and the orchard is outside the walls, the walls do not serve to make the orchard surrounded for residential purposes.