The Bavli, Bava Basra 6:2, discusses the case that someone's rooftop is adjacent to another person's courtyard floor, and then the case where the rooftop is actually lower (but not much lower) than the courtyard floor.
In the first case, it indicates that the rooftop's owner must pay for a four-cubit-tall barrier between his roof and his neighbor's yard, so that he cannot see what's going on in his neighbor's yard.
In the second case, it indicates that the two neighbors must chip in on a ten-handbreadth-tall barrier (much shorter than four cubits) between the roof and the yard, so as to prevent either neighbor from accidentally overstepping their common boundary. The Rosh adds that the rooftop's owner must then pay, by himself, to build that barrier up to four cubits, so he cannot see what's transpiring in the courtyard.
Logically, then, it seems to be that the Rosh should hold — as, indeed, Shulchan Aruch (160:1) later holds — that in the first case, where the rooftop and courtyard are on a level, the lowest ten handbreadths of the wall should be paid for by both neighbors. But he doesn't say so, and the g'mara doesn't say so. Does he, in fact, hold that way?
 More precisely: to make overstepping the boundary noticeable to the one doing so. But that's neither here nor there.