The cathedral built by Bartolommeo Ammanati (1S70), modified by Ippolito Scalza, and completed in 1680 (with the exception of the facade, which is still unfinished) contains a large altar-piece by Taddeo di Bartolo of Siena, and the fragments of an imposing monument erected in1427-1436by the Florentine architect Michelozzo in honour of Bartolommeo Aragazzi, secretary of Pope Martin V., which was taken down in the 18th century.
He made his brother, Antonio, a distinguished soldier, and two nephews, cardinals, and gave to a third nephew, Taddeo, the principality of Palestrina.
Great jealousy of their increasing power was excited amongst the neighbouring princes, and Odoardo Farnese, duke of Parma, made war upon Taddeo, and defeated the papal troops.
Taddeo fled to Paris, where he died in 1647, and with him the family became extinct in the male line.
But the struggles between Guelphs and Ghibellines in Bologna itself soon followed, and the commune was so weakened that in 1337 Taddeo de' Pepoli made himself master of the town, and in 1350 his son sold it to Giovanni Visconti of Milan.
In the Sala dei Nove or della Pace above are the noble allegorical frescoes of Ambrogio Lorenzetti representing the effects of just and unjust government; the Sala delle Balestre or del Mappamondo is painted by Simone di Martino (Memmi) and others, the Cappella della Signoria by Taddeo di Bartolo, and the Sala del Consistorio by Beccafumi.