It presently houses the Museo di Roma, the "Museum of Rome", covering the history of the city in the period from the Middle Ages through the nineteenth century.

He was the son of Count Girolamo Onesti and Giulia Braschi, sister of Pope Pius VI, who created him Duke of Nemi.

[2] Construction was suspended in February 1798 during the Napoleonic occupation of the city, when the French temporarily took possession of it until 1802 and confiscated the recently acquired collection of antiquities it contained (though Braschi was reimbursed for them).

In 1871 the Braschi Onesti heirs sold the building to the Italian State, who made it the seat of the Ministry of Interior (now moved into Palazzo del Viminale).

The oval hall inside the main entrance overlooks Via San Pantaleo, and leads to the monumental staircase with its eighteen red granite columns which came from the gallery built by the Emperor Caligula on the banks of the River Tiber.