The area of ​​the present square coincides with the area where the oldest nucleus of the city developed, the Borgo al Cornio, already documented in the ninth century and probably of Lombard foundation, where there was a small village and the church dedicated to Saint Stephen. The initially modest structures that made up the village, probably in wood, were soon transformed into masonry structures, which already occupied the area of ​​the present square in the 11th century. The square, wide and articulated, has developed in function of the baptismal church which later became the Cathedral. Since the fourteenth century the square was the scene of festivals, markets, outdoor games and above all the Display of the Sacred Belt that attracted, as today, crowds of faithful people devoted to the Marian relic.
The square is surrounded by buildings mostly of fourteenth-century origins. On the south side is the nineteenth-century Palazzo Vestri, in front of which stands the statue of Giuseppe Mazzoni, a Prato politician, realized by Alessandro Lazzerini. Next to the Cathedral is the Bishop's Palace and in front of it the Dragoni Palace, renovated in the nineteenth century.
In the middle of the square is the nineteenth-century Fontana del Pescatorello named "Duck's fountain" (1863) sculpted by Emanuele Caroni and Ulisse Cambi.