The exquisite Baroque church of San Carlo alle Quattro Fontane (St Charles at the Four Fountains), or San Carlino as it is better known, is the work of one of most original minds in the history of architecture, Francesco Borromini (1599-1667).
San Carlino, which is jointly dedicated to St Charles Borromeo and the Holy Trinity, was commissioned by the Spanish Trinitarians, a religious order whose mission was to help free Christians captured and enslaved by Muslim pirates. Work began in 1634. Faced with the challenge of working with a very limited space, Borromini's first task was to create the monastic buildings and the cloister. One of the most surprising aspects of the design of the latter can be seen in the corners, which should be supported by columns, but which Borromini leaves open.
He then turned his attention to the church, creating one of the most extraordinary spaces in Rome. The layout is a complex interplay of convex and concave surfaces using the equilateral triangle as a unifying device. The symbolism throughout is based on the Holy Trinity.
The altarpiece at the high altar depicts St Charles Borromeo, St John of Matha and St Felix of Valois Venerating the Holy Trinity, the work of a French artist Pierre Mignard (1612-95), also known as Mignard le Romain.
The oval dome is, without doubt, the glory of the church. The complex coffering in the cupola is made up of crosses, octagons and hexagons, which decrease in size towards the oculus. The inscription around the base of the lantern reads: SANCTISS · TRINITATI · BEATOQ · CAROLO · BORROMEO · D · AN · SAL · MDCXI.The lantern ceiling sports an image of the Dove of the Holy Spirit within a triangle, both of which are contained within a circle, which is, itself, set within an octagonal blaze of glory. The four pendentives have tondi with stucco reliefs by Giuseppe and Giulio Bernasconi, which illustrate scenes from the foundation of the Trinitarian order: The Meeting of St Felix and St John of Matha, Pope Innocent III Approves the Order, The Founders Receive the Habit and The First Ransoming of Captives.
San Carlino was consecrated in 1646, but its extraordinary undulating facade wasn't built until after Borromini's death. It was constructed, to Borromini's design, by his nephew Bernardo, between 1670 and 1680, contrary to the date in the inscription. The entrance is surmounted by a statue of St Charles Borromeo, the work of Antonio Raggi (1624-86), a pupil of Gian Lorenzo Bernini. The statue is flanked by a pair of angels, who hold up two wings to form an arch. To either side are statues of St John of Matha andSt Felix of Valois, the founders of the Trinitarian Order, which were executed by Sillano Sillani.