Yosef Karo

“ This is true if the walls of the alley continue a bit into the yard such that the protrusion is not apparent to one standing in the alley. However, if they are visible, they are considered stakes and the alley is permitted. If the residents of the yard made an eruv with the residents of the alley and the alley's opening to the yard is not at the side of the yard, and the opening of the yard on the other side [towards the public domain] is not opposite the opening to the alley, and the yard is public, the alley is also permitted. ”