Two major commissions at this period result in Poussin's early masterwork, the Barberini Death of Germanicus (1628), partly inspired by the reliefs of the Meleager sarcophagus, and the commission for St. Peter's that amounts to a public debut, the Martyrdom of St. Erasmus (1629, Vatican Pinacoteca), indebted to designs on the same subject by the contemporary Baroque painter, Pietro da Cortona.

Pietro Berrettini da Cortona, born to a family of artisans including his uncle Filippo Berrettini, in Cortona, at this time a town in the Grand Duchy of Tuscany, had first apprenticed with Andrea Commodi in Florence but soon departed for Rome at about 1612, where he had joined the studio of Baccio Ciarpi.

In Rome, he had received encouragement from many prominent patrons including the Colonna.

According to a biography, his deft copies of Raphael's Roman frescoes had brought him to the attention and patronage (1623) of the Sacchetti brothers, Marcello and Giulio, who have become respectively papal treasurer and cardinal (1626) during the Barberini papacy.

In the Sacchetti orbit, he has met Cardinal Francesco Barberini, the nephew of Pope Urban VIII, as well as the antiquarian, Cassiano dal Pozzo.

These three men had helped him gain a major commission in Rome (1624-1626), a fresco decoration in the newly constructed Bernini church of Santa Bibiana.

In 1626, the Sacchetti had engaged Cortona to paint for them three large canvases of the Sacrifice of Polyxena, Triumph of Bacchus, and Rape of the Sabines (the latter, in about 1629), and to paint a series of frescoes in the Villa Sacchetti in Castel Fusano, near Ostia, using a team that includes the young Andrea Sacchi.

Soon the rising prodigy will attract the patronage of the powerful papal Barberini family.

He has already been involved in the fresco decoration of the Palazzo Mattei,and Cardinal Orsini had commissioned from him an Adoration of the Shepherds around 1626 for San Salvatore in Lauro.

Poussin, having fallen ill, had been taken into the house of his compatriot Jacques Dughet, where he had been nursed by Dughet’s daughter, Anna Maria, who Poussin marries in 1630.

His two brothers-in-law are artists and a nephew, Gaspard Dughet, will later took Poussin’s surname.

During the late 1620s and 1630s, Poussin has the opportunity to experiment and formulate his own stance in painting with reference to others which he does through study of the antique and works such as Titian’s Bacchanals at the Casino Ludovisi and the paintings of Domenichino and Guido Reni.

At the same time, the Roman Baroque is emerging: in the 1620s, Cortona was producing his early Baroque paintings for the Sacchetti family; Bernini, having established his reputation in sculpture, was designing the great bronze baldachin in St. Peter’s; and an ingenious architectural imagination was emerging in works by Borromini.

Bernini completes work on the Palazzo Barberini by 1633.

The palazzo is disposed around a forecourt centered on Bernini's grand two-story 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 protects the giardino segreto ("secret garden"), from the piazza below, above which it rises from a rusticated basement that is slightly battered like a military bastion.

The main block presents three tiers of great arch-headed windows, like glazed arcades, a formula that is more Venetian than Roman.

On the uppermost floor, Borromini's windows are set in a false perspective that suggests extra depth, a feature that will be copied into the twentieth 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, or helicoidal, staircase by Borromini to the right.

Aside from Borromini's false-perspective window reveals, among the other influential aspects of Palazzo Barberini, ones that will be repeated throughout Europe, are 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.

The Salon ceiling is graced by Pietro da Cortona's masterpiece, the Baroque fresco of the Allegory of Divine Providence and Barberini Power.

This vast panegyric allegory is to become highly influential in guiding decoration for palatial and church ceilings; its influence can be seen in other panoramic scenes such as the frescoed ceilings at Sant'Ignazio (by Pozzo); or those at Villa Pisani at Stra, the throne room of the Royal Palace of Madrid, and the Ca' Rezzonico in Venice (by Tiepolo).

Also in the palace is a masterpiece of Andrea Sacchi, a contemporary critic of the Cortona style, Divine Wisdom.

Today Palazzo Barberini houses the Galleria Nazionale d'Arte Antica, one of the most important painting collections in Italy, and is home also to the Italian Institute of Numismatics.

Recently found hidden in the cellars of the rear part of the building is a Mithraeum, dating probably from the second century CE.

Most of the fresco cycles so numerous in early seventeenth century-Rome represent framed episodes imitating canvases such as found in the Sistine Chapel ceiling or in Carraccis' The Loves of the Gods in the Farnese gallery, completed in 1601.

Pope Urban VIII (Maffeo Barberini) had in 1633 commissioned Pietro da Cortona to paint a fresco cycle for the ceiling of their family palace.

The huge Allegory of Divine Providence and Barberini Power, completed six years later, marks a watershed in Baroque painting.

Containing endless number of heraldic symbols and subthemes, the fresco is an illusion with the central field apparently open to the sky and scores of figures seen 'al di sotto in su' apparently coming into the room itself or floating above it.