This week, Daf Yomi readers began a new tractate, Bava Batra, which begins by considering these same questions about walls. Who builds them, who is responsible for them, and what purpose do they serve? ...Timely, insofar as the subject is walls and neighbors, but not all that informative about the issue in the headline.
So far, matters are entirely straightforward. The difficulty, and the source of legal interest, arises when one partner wants to build a wall and the other partner does not. Can one compel the other to build the wall, or to pay for its construction? The Gemara points out that what is at stake here is whether there is such a thing as a right to privacy. Does one neighbor have the right to be shielded from the observation of the other neighbor? If he does, then he can force the latter to build a wall, even against his will?
Earlier Daf Yomi columns are noted here and links.