Built in the 12th century on the remains of the ancient Tabularium and using pre-existing fortifications built by powerful baronial families, this palazzo makes up the background to the piazza. It owes its name to its function as senatorial seat responsible for the administration of justice.
In 1299 an open loggia was added overlooking the piazza, although this was walled in a century later in order to strengthen the Palazzo, which was turned into a fortress with the addition of towers by Pope Boniface IX; more corner towers were later added by Popes Martin V and Nicholas V.
Michelangelo also designed a monumental double-flighted staircase for the Palazzo Senatorio, in addition to dividing up the façade with gigantic pilaster strips, large windows and tympanums and a crowning balustrade featuring statues.
The work was completed after his death by Giacomo della Porta, who made some alterations to Michelangelo's plan. The bell-tower was also the result of an alteration to the original plan, this time by Martino Longhi the Elder. The entrance to Palazzo Senatorio is in Piazza del Campidoglio at the corner with via S. Pietro in Carcere. The tour includes the Studio and the blue salon of the Mayor with a view on the Forum, the Hall of the Tapestries, the Hall of the Banners, the Julius Caesar hall, seat of the city council, and the Collection of busts. From here, through the portico by Vignola, you enter the Picture Gallery and the Roman Garden with the Exedra of Marcus Aurelius. Handicapped visitors can use the lift of the Tabularium (entrance in via di San Pietro in Carcere) to reach the first floor of the Palazzo Senatorio, and they will follow the other visitors. No booking is needed.