One of the famous baroque churches for its scenographic setting and for its solemnity, the church of St Charles of the Four Fountains is often unjustly forgotten. It is, in fact, one of the most original works by Borromini, ingeniously carried out with the same dimensions as one of the pilasters of the dome of St Peter. The church, founded in 1638 and incorporated in the convent of the Spanish Trinitarians, who commissioned the work, is Borromini’s last work, left incomplete upon his death (1667). The high and narrow winding façade is divided into six parts and has a portal surmounted by a niche with St Charles in a praying position. The interior, with an oval plan and covered by an elliptical dome elegantly decorated by hexagons, octagones and crosses that gradually reduce towards the little lantern, houses Corinthian columns that follow the winding movements of the walls, which house gracious chapels. The small cloister appears harmonic, organic, and proportioned: it is surrounded by Dorian and coupled columns that support the arches of the elegant porch.