The city's most impressive museum occupies rooms richly frescoed by artists of the Sienese school. These frescoes are unusual in that they were commissioned by the governing body of the city, rather than by the Church, and many depict secular subjects instead of the favoured religious themes of the time.

Upstairs, start in the Sala del Risorgimento with its impressive late-19th-century frescoes serialising key events in the campaign to unite Italy. Next is the Sala di Balia (or Sala dei Priori). The 15 scenes depicted in frescoes around the walls recount episodes in the life of Pope Alexander III (the Sienese Rolando Bandinelli), including his clashes with the Holy Roman Emperor Frederick Barbarossa. You then pass into the Anticamera del Concistoro, remarkable for the fresco (moved here in the 19th century) of Saints Catherine of Alexandria, John and Augustine, executed by Ambrogio Lorenzetti. The next hall, the Sala del Concistoro, is dominated by the allegorical ceiling frescoes of the Mannerist Domenico Beccafumi. Back in the Anticamera del Concistoro, you pass to your right into the Vestibolo (Vestibule), whose star attraction is a bronze wolf, the symbol of the city. Next door in the Anticappella are frescoes of scenes from Greco- Roman mythology and history, while the Cappella (Chapel) contains a fine Holy Family and St Leonard by Il Sodoma and intricately carved wooden choir stalls.

The best is saved for last, though. From the Cappella, you emerge into the Sala del Mappamondo, where you can admire the museum's masterpiece, Simone Martini's powerful and striking Maestà (Virgin Mary in Majesty). Completed in 1315, it features the Madonna beneath a canopy surrounded by saints and angels and is Martini's first known work. On the other side of the room is another work attributed to Martini, his oft-reproduced fresco (1328–30) of Guidoriccio da Fogliano, a captain of the Sienese army.

The next room, the Sala della Pace, is where the Council of Nine was based. It is decorated with Ambrogio Lorenzetti's fresco cycle known as the Allegories of Good and Bad Government (c 1338–40). The central allegory portrays scenes with personifications of Justice, Wisdom, Virtue and Peace, all unusually (at the time) depicted as women, rendered along with scenes of criminal punishment and rewards for righteousness. Set perpendicular from it are the frescoes Allegory of Good Government and Allegory of Bad Government, which feature intensely contrasting scenes set in the recognisable environs of Siena. The good depicts a sunlit, idyllic, serene city, with joyous citizens and a countryside filled with crops; the bad city is filled with vices, crime and disease. These frescoes are often described as the most important secular paintings of the Renaissance, and shouldn't be missed.

Finish by backtracking and climbing the stairs to the loggia, which looks southeast over Piazza del Mercato and the countryside.