Story of Narrow Alleys
Aside from the thick blue lines which indicate “roads”, the thin blue lines are generally alleys. Some alleys were wide enough to accomodate street vendors who sold various fried snacks (you tiao or fried bread, for example). Other alleys were narrow enough to fit only two persons through, shoulder to shoulder.
Some of the more affluent families had the wealth to tear down their wooden houses and built new structures with cement. Their original houses tended to be humble, sharing yard spaces with neighbors. In the process of building new houses, because it was unclear to whom the yard spaces below, at times new houses were built into these yards. In rare cases like these, newer houses tended to expand into the alleys, making them even narrower. Most houses remained their original conditions, however.
Some alleys are impossible to be occupied by newer houses. For instance, some alleys had a drainage system underneath its stone block foundation. These stone blocks cannot sustain the weight of cement houses, so were spared from the expansion of few new houses.