692. A City Within One's Techum

95:11 If one placed his eiruv in the middle of a city, then the whole city is considered his "home base" for Shabbos and he may traverse the entire city even if it's very large, as well as the outskirts and the Shabbos boundary, because it's considered as if he is residing in that city.

95:12 If within a place's Shabbos boundary there is a walled city or a city that has an eiruv, this city is not measured. Instead, it is only counted as four cubits so long as the Shabbos boundary reaches beyond the city. (It need not be beyond per se, but it must reach the end of the city - Mishnah Brurah 408:9.) For example, if from one's eiruvei techumin to the city is 500 cubits and the length of the city is 1,000 cubits (total: 1,500) then the city is only counted as four cubits and we have from outside the city an additional 1,496 cubits to the Shabbos boundary (1,500 - 4 = 1,496). There's no difference between the side of the Shabbos boundary that's before one's eiruv and the side that's after it. However, if one's Shabbos boundary ends in the middle of the city, he may not walk any further because it would be beyond his boundary. In this case, it's not considered as if the whole city is only four cubits.