Construction of the palace started in 1627 by architect Maderno who was assisted by his nephew Borromini. After Maderno death it as continued by Bernini and was completed in 1633.
"The palazzo is around a forecourt centered on Bernini's two-story hall backed by an oval salon, 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. 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 to the right."
The palace contains Borromini's false-perspective window reveals, the unit of a central two-story hall backed by an oval salone and the symmetrical wings that extended forward from the main block to create a cour d'honneur, which is a three sided courtyard.
The salon ceiling is graced by Pietro da Cortona's masterpiece, the Baroque fresco of the Allegory of Divine Providence and Barberini Power. . Also in the palace has a masterpiece by Andrea Sacchi, a contemporary critic of the Cortona style, Divine Wisdom.
The rooms of the piano nobile have frescoed ceilings by other seventeenth-century artists like Giuseppe Passeri and Andrea Camassei, plus, in the museum collection, precious detached frescoes by Polidoro da Caravaggio.
Today, the palace is the Museum of Ancient Art which houses a collection of Renaissance and Baroque paintings.
http://www.youtube.com/watch?v=pezC87bwWOE (in Italian but good virtual tour)