Both these entrances lead into a large courtyard on one side of which is a two level open arcade, with paired Doric and Ionic columns, that frames the garden beyond.

The architectural historian Howard Hibbard has demonstrated that the nine-bay section of the palace on Piazza Fontanella Borghese was begun in 1560/61 for Monsignor Tommaso del Giglio, whose coat of arms or stemma remain over the door in Piazza Borghese, and he suggests that the architect was Vignola,[1] an attribution accepted by Anthony Blunt,[2] considered conclusive by James S. Ackerman[3] and followed by other scholars since, with more or less reduced interventions by Longhi.

Before Tomasso del Giglio died in 1578, the façade and the unique but undocumented double-columned two-storey arcade of the courtyard had been established, a conceptual scheme of Vignola's, Hibbard suggested, but which was carried out in 1575-78 by Martino Longhi the Elder.

[5] In 1671-76 Carlo Rainaldi added new features for Prince Giovan Battista Borghese; the most extensive changes were made on the newly raised ground floor of the long wing extending towards the Tiber, ending with river views, which the Borghese found the most congenial dwelling spaces: Rainaldi added the columnar loggia to Ponzi's end facade (illustration, right), and on the interior a richly stuccoed oval chapel, and the narrow barrel-vaulted galleria, the highly charged Cortonesque decorative details of which were designed by Giovan Francesco Grimaldi (1606–1680).

The edifice has a magnificent inner courtyard, surrounded by ninety-six granite columns and decorated with statues, a nympheum, and other features, as well as an enclosed garden with three niche wall-fountains built to designs by Johann Paul Schor and finished by Rainaldi for Prince Giovan Battista Borghese in 1673.