The Church of San Carlo alle Quattro Fontane, affectionately nicknamed "San Carlino" due to its small size, represents one of the greatest masterpieces of Francesco Borromini and of Baroque architecture as a whole.

Built together with the cloister and convent between 1634 and 1644, the complex was commissioned by the Spanish Order of the Discalced Trinitarians and dedicated to Saint Charles Borromeo. The façade is a triumph of dynamism, defined by alternating concave and convex lines and enriched by a decorative apparatus of angels, palm branches, and a characteristic canopied window.

Inside, the plan intertwines various geometric figures culminating in the dome, a true masterpiece of Borromini: the "honeycomb" ceiling It features crosses, hexagons, and octagons that taper toward the center, creating an illusion of extraordinary depth. The elegance of the architectural lines more than compensates for the lack of opulent decoration. Finally, the tiny octagonal cloister with double loggia is a model of harmony, where Borromini manages to transform the small space into a welcoming and airy environment.