Francesco Borromini built the church of San Carlo alle Quattro Fontane between 1634 and 1641, giving free rein to all his talent. The greatest merit of San Carlino is that Borromini started from two strong constraints:

- The small and irregular space. The church of San Carlino would fit inside one of the four pillars supporting the dome of St. Peter's Basilica in the Vatican; hence the popular saying: «San Pietro è bello per la sua grandeza, e San Carlino per la sua piccolezza» (St. Peter's is beautiful for its grandeur, and San Carlino for its smallness).
- The building materials were poor, or rather, the economic resources were scarce. The entire building is made of solid brick, rendered and painted in the same colour range, and the original floor was made of terracotta bricks. The architectural lines drawn by Borromini made up for and even surpassed in beauty the lack of ornamentation.

It is therefore understandable that whoever enters San Carlino imagines himself inside a sublime sculpture and delights in admiring its unspeakable beauty.

Borromini started from a Greek cross plan, with the longest longitudinal section, and replaced the rectilinear forms with concave and convex curves.

The original floor of the church was made of terracotta bricks; they were covered in 1898 by the marble paving that can be seen today.

The first level is made up, in the purest Madernian style, of gigantic, smooth columns, crowned with capitals of acanthus leaves, volutes and representations of pomegranates and laurels, the latter symbolising the Passion and victory.

The columns are arranged in four series of four columns each, which are placed over the four concavities of the church's perimeter. In each of these series, between the columns, there is a central doorway and niches, figuratively representing four façades facing the interior of the church.

The second level of the structure is located between the cornice supported by the columns and the dome. It is undoubtedly the most technically difficult level, as the elliptical dome has to sit on the sinuous ground plan of the church.

Borromini achieved this architectural feat by using four lateral arches and, behind them, four apses, adorned with coffered ceilings and rose windows and with a pediment at their base. It is noteworthy that the two side apses are slightly flattened, although this alteration is hardly noticeable due to the foreshortened design of the coffers.

In order to support the dome and create architectural beauty, Borromini resorted to twisting the two lateral arches as they descend to their base.

The pendentives are adorned with medallions with relief images of the life of Saint John of Matha, founder of the Trinitarian Order. These medallions are bordered by winged cherub faces, so characteristic of the Borrominian style.

The third level corresponds entirely to the elliptical dome of the church, embellished by a large lantern at its apex. At its base, the dome is adorned with a laurel wreath and a noble crown; at the summit, the figure of the dove of the Holy Spirit appears in relief. Thus, victory and royalty are symbolised at the base, and glory at the summit.

The rest of the dome is adorned with geometric figures in relief, with the Greek cross, typical of the iconography of the Trinitarian Order, standing out. The lantern and two windows at the base of the dome illuminate the whole church.

The lighting of San Carlino is strongly conditioned by the intensity of the sunlight. The zenithal light is diffused or profuse depending on its reflection on the walls of the church, which are soft and uniformly coloured.

The main altar is presided over by a beautiful fresco of St. Charles Borromeo, the 16th century Italian cardinal to whom the church is dedicated, and the founders of the Trinitarian Order in adoration of the Holy Trinity, the work of Pierre Mignard.

On the side altars there are two large canvases corresponding to the 'Ecstasy of St. John the Baptist of the Conception', by Prospero Mallerini; and the 'Ecstasy of St. Michael of the Saints', by Amalia de Angelis.

Under the altar is the urn with the remains of Blessed Isabel Canori Mora, mother of the family and Trinitarian Tertiary.

To the left of the high altar is the chapel of the Barberini family, which is evidenced by the coat of arms of the three bees typical of this noble Roman family. The painting Rest of the Holy Family in Egypt by Giovanni Francesco Romanelli presides over the chapel.

The Chapel of the Passion is to the right of the entrance to the church. It is decorated with three paintings by Giuseppe Milanese: The Crowning of Thorns, The Scourging and The Crucifixion.