Eruvin 25a and Eruvin 25b, ways to rectify an area too large to be used with walls that were not originally placed there to make it a living space, either by reducing the space or by building a new wall, and how distinct it has to be. Or, by a pi tikrah yored ve-sotem, the lip of a ceiling can count as a wall coming down, and then when a karfaf borders on human inhabited spaces and how those work.
