Unlike Bernini who easily adopted the mantle of the charming courtier in his pursuit of important commissions, Borromini was both melancholic and quick in temper which resulted in his withdrawing from certain jobs.

San Carlino is remarkably small given its significance to Baroque architecture; it has been noted that the whole building would fit into one of the dome piers of Saint Peter's.

Although the idea for the serpentine façade must have been conceived fairly early on, probably in the mid-1630s, it was only constructed towards the end of Borromini's life and the upper part was not completed until after the architect's death.

Illuminated by windows hidden from a viewer below, interlocking octagons, crosses and hexagons diminish in size as the dome rises to a lantern with the symbol of the Trinity.

The white oratory interior has a ribbed vault and a complex wall arrangement of engaged pilasters along with freestanding columns supporting first-level balconies.

Inside, the nave has an unusual centralized plan circled by alternating concave and convex-ending cornices, leading to a dome decorated with linear arrays of stars and putti.

The geometry of the structure is a symmetric six-pointed star; from the centre of the floor, the cornice looks like two equilateral triangles forming a hexagon, but three of the points are clover-like, while the other three are concavely clipped.

The fusion of feverish and dynamic baroque excesses with rationalistic geometry is an excellent match for a church in a papal institution of higher learning.

The decision to rebuild the church was taken in 1652 as part of Pope Innocent X's project to enhance the Piazza Navona, the urban space onto which his family palace, the Palazzo Pamphili, faced.

The foundations were laid and much of the lower level walls had been constructed when the Rainaldis were dismissed due to criticisms of the design and Borromini was appointed in their stead.

[18] Borromini began a much more innovative approach to the façade which was expanded to include parts of the adjacent Palazzo Pamphili and gain space for his two bell towers.

In 1668, Carlo Rainaldi returned as architect and Ciro Ferri received the commission to fresco the dome interior which it is highly unlikely that Borromini intended.

Further large-scale statuary and coloured marbling were also added; again, these are not part of Borromini's design repertoire which was orientated to white stucco architectural and symbolic motifs.

Borromini's works include: In the summer of 1667, and following the completion of the Falconieri chapel (the High Altar chapel) in San Giovanni dei Fiorentini, Borromini committed suicide in Rome by drawing his sword, resting the hilt against his bed, and falling on it "with such force that it ran into [his] body, from one side to the other"[23] This was possibly as a result of nervous disorders and depression.

In his testament, Borromini wrote that he did not want any name on his burial and expressed the desire to be buried in the tomb of his kinsman Carlo Maderno in San Giovanni dei Fiorentini.