The staircase, designed by Francesco Borromini, serves the south wing of the building, and is complementary to that of Bernini, according to a principle that is not only aesthetic, but also distributive and functional. Accessible from the external portico, it led to the rooms of Cardinal Francesco Barberini and was intended for a more private circulation. It is helical, so it follows the principle of screwing around an axis of rotation, and has an oval plan, that is, it has a longitudinal flattening, allowing an easier climb than circular stairs. This model is codified in the sixteenth-century treatises by Vignola, Sebastiano Serlio and Andrea Palladio. Each turn is made up of 12 coupled Doric columns, whose capital is decorated with small bees (heraldic symbol of the family). The major axis measures 9.40 m, the minor axis 7.85 m. The light enters from the oculus at the top and from the windows of the facade. In the initial project, the staircase ended with a free ramp, then it is later raised above the top floor of the building to house the rich library of Cardinal Francesco, now transferred to the Vatican. The spiral structures, with all the design problems that derive from them, are particularly congenial to the eccentric spirit of Borromini, who re-proposes them in numerous drawings and in architectural details of other works.