The historic center of Bosa has maintained its appearance over the centuries. By building type, three phases of development can be distinguished, giving rise to three neighborhoods with different architectural peculiarities: the medieval village Sa Costa, an ancient part that can be traced back to the 13th century; the free town Corte Intro; and the 19th-century settlement Sa Piatta, where citizens liked to meet. In a tempera from the 1600s, the city now appears complete and welded to the castle by a mighty city wall, which protected the settlement all the way to the river. The village of Sa Costa is the one that, starting from the castle, extends to Carmine Street. It consists of four main streets that follow the elevation curves of the hill and several cobbled lanes, with trachyte steps interrupting the horizontal path. It is bounded by two long stairways called S’Iscala Longa to the west near Piazza Carmine and S’Iscala e sa Rosa to the east accessed from Piazza Episcopio. The colorful buildings, have two overlooks one upstream and the other downstream and are characterized by one room per floor, pink trachyte portals often bearing numerous symbolic motifs and friezes. In the lower part of the city, almost in “line” with the river is the urban area referred to as Intro Court, characterized by courtyard-like widenings where trade was once practiced. Here the buildings are taller and some continue to maintain one room per floor, the doorways are usually arched, and the roof is gabled. Various alleys finally lead to Corso Vittorio Emanuele, a place of great vitality.