If thou wouldst know who are these two,
the valley whence the Bisenzio descends
belonged to their father Albert and to them.
They issued from one body; and thou mayst search
all Caina, and thou wilt not find shade
more worthy to be fixed in ice
(Dante, The Divine Comedy, Inferno. Canto XXXII, 55-60)
The powerful Alberti family, of Longobard or Frankish origin, played an extremely important role in medieval Prato.
At the beginning of the 11th century, Ildebrando degli Alberti owned a court and a palace in the location where the palatium imperatoris would later be erected, that is, the castle that in the following century, together with the nearby Borgo al Cornio (situated near the current Piazza del Duomo), would form the first nucleus of the future city. Prato, and later the Bisenzio Valley (where the Alberti family at the beginning of the 12th century built the Fortress of Cerbaia, to control the "Lombardy" route towards the Apennines ridge), became the main centre of the noble family.
At the height of their power, in the 11th century, they were made counts of Prato through imperial investiture and came to own extensive lands between the Apennines and the Maremma region; they later broke up into branches corresponding to the family's different fiefdoms
Their decline began in the early 12th century: in 1107, during the war that pitted the Alberti family against Matilde of Canossa, Prato was besieged and destroyed by the marquise's troops, allied with those of Pistoia, Florence and Lucca. After that defeat the Alberti family had to give up their ambition of making of Prato a power capable of standing up to nearby cities and particularly to Florence.