“What is a reshus harabbim? A street or marketplace that … is not walled and even if they are walled but they [the street or marketplace] are open from city gate to city gate, they [the street or marketplace] would be considered a reshus harabbim ….”
Even though the Shulchan Aruch cites the criterion of mefulash m’shaar l’shaar in conjunction with a walled city, the Tur does not differentiate between a walled city and an open city and the Bais Yosef (siman 345) does not disagree with the Tur. Therefore, mefulash would be considered a criterion of a reshus harabbim even in a city that is not walled (see Bais Av, 2:9:1 for a detailed explanation). The Gra offers a different perspective (Bi’ur HaGra, O.C. 345:7) and explains that the words of the Shulchan Aruch, “is not walled,” refers to a street [sratya]. The Dmesek Eliezer clarifies the Gra that a sratya is a road which does not have a wall around it. The Gra explains further that, “even if they are walled,” refers to a marketplace [platya]. The Dmesek Eliezer explains that a platya is a city street that is walled on two sides and the remaining two sides are open straight from city gate to city gate. What we see from this Dmesek Eliezer is that what the Shulchan Aruch refers to as, “walled,” pertains to the streets of the city and not the city walls. Therefore, it is understandable why the Gra (Sh’nos Eliyahu, introduction to Meseches Shabbos and Chidushi HaGra, Shabbos 6a) when discussing the criterion of mefulash m’shaar l’shaar does not attribute it to a walled city. It follows that our streets which are lined with houses would be classified as walled streets and would have to be mefulashim u’mechuvanim m’shaar l’shaar to be categorized as a reshus harabbim. Note that since the term city gate [shaar] does not always pertain to the gate of a walled city (Rashi, Yoma 11a), mefulash m’shaar l’shaar can refer to a city that is not walled as well.