It is an iconic masterpiece of Baroque architecture, built as part of a complex of monastic buildings on the Quirinal Hill for the Spanish Trinitarians, an order dedicated to the freeing of Christian slaves.

Although the idea for the serpentine facade 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.

The inscriptions found in San Carlo, a valuable source illustrating the history of the church, have been collected and published by Vincenzo Forcella.

Borromini fled to Rome to avoid his debt and found himself becoming an architectural star pupil under renowned Italian architect Carlo Maderno.

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 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.

[2] The rise of baroque architecture prompted Borromini to bring his sculpture background to life by creating unexpected combinations of curves and rectangular forms in his work.

The arches which spring from the diagonally placed columns of the lower wall order to frame the altars and entrance, rise to meet the oval entablature and so define the space of the pendentives in which roundels are set.

The oval entablature to the dome has a 'crown' of foliage and frames a view of deep set interlocking coffering of octagons, crosses and hexagons which diminish in size the higher they rise.

Further complexity is introduced by the variation in the spacing of the twelve columns carrying alternating round and flat headed openings, the curvature of the corners, and the inventive balustrade.