The concave-convex facade of San Carlo undulates in a non-classic way.
Tall corinthian columns stand on plinths and bear the main entablatures; these define the main framework of two storeys and the tripartite bay division.
Between the columns, smaller columns with their entablatures weave behind the main columns and in turn they frame niches, windows, a variety of sculptures as well as the main door, the central oval aedicule of the upper order and the oval framed medallion borne aloft by angels.
The medallion once contained a 1677 fresco by a Pietro Giarguzzi of the Holy Trinity.
Above the main entrance, cherubim herms frame the central figure of Saint Charles Borromeo by Antonio Raggi and to either side are statues of St. John of Matha and St. Felix of Valois, the founders of the Trinitarian Order.

The plan and section show the layout of the cramped and difficult site; the church is on the corner with the cloister next to it and both face onto the Via Pia.
The monastic buildings straddle the site, beyond which Borromini intended to design a garden.
The church itself is only 66 feet wide and 39 feet long – deeming a non-classic plan vital.
Before the baroque style became the style of choice in the seventeenth century, the renaissance was full of buildings with very rectangular shapes.
The baroque style introduced curves and ovals, which Borromini combined with cubic elements to create a floor plan that fit the awkward site’s requirements.
Not only did the curves suit the site, but it also enhanced the baroque design by creating movement, playing with lighting and shadows, adding ornament, and extra drama.
The concave-convex facade of San Carlo undulates in a non-classic way. Tall corinthian columns stand on plinths and bear the main entablatures; these define the main framework of two storeys and the tripartite bay division. Between the columns, smaller columns with their entablatures weave behind the main columns and in turn they frame niches, windows, a variety of sculptures as well as the main door, the central oval aedicule of the upper order and the oval framed medallion borne aloft by angels. The medallion once contained a 1677 fresco by a Pietro Giarguzzi of the Holy Trinity. Above the main entrance, cherubim herms frame the central figure of Saint Charles Borromeo by Antonio Raggi and to either side are statues of St. John of Matha and St. Felix of Valois, the founders of the Trinitarian Order. 
The plan and section show the layout of the cramped and difficult site; the church is on the corner with the cloister next to it and both face onto the Via Pia. The monastic buildings straddle the site, beyond which Borromini intended to design a garden. The church itself is only 66 feet wide and 39 feet long – deeming a non-classic plan vital. Before the baroque style became the style of choice in the seventeenth century, the renaissance was full of buildings with very rectangular shapes. The baroque style introduced curves and ovals, which Borromini combined with cubic elements to create a floor plan that fit the awkward site’s requirements. Not only did the curves suit the site, but it also enhanced the baroque design by creating movement, playing with lighting and shadows, adding ornament, and extra drama.
San Carlo alle Quattro Fontane