The palazzo is disposed around a forecourt centered on Bernini's grand two-storey hall backed by an oval salone, with an extended wing dominating the piazza, which lies on a lower level. At the rear, a long wing protected the garden from the piazza below, above which it rose from a rusticated basement that was slightly battered like a military bastion. The main block presents three tiers of great arch-headed windows, like glazed arcades, a formula that was more Venetian than Roman. On the uppermost floor, Borromini's windows are set in a false perspective that suggests extra depth, a feature that has been copied into the 20th century. Flanking the hall, two sets of stairs lead to the piano nobile, a large squared staircase by Bernini to the left and a smaller oval staircase by Borromini to the right.
The famous helicoidal staircase by Borromini.
As well as Borromini's false-perspective window reveals, other influential aspects of Palazzo Barberini that were repeated throughout Europe include the unit of a central two-storey hall backed by an oval salone and the symmetrical wings that extended forward from the main block to create a cour d'honneur.