The small Baroque church of San Carlo alle Quattro Fontane, 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).
The church was commissioned in 1634 by the Spanish Trinitarian Fathers, a religious order whose mission was to help free Christians captured by Muslim pirates.
Borromini's first task was to build the small cloister and he came up with a design that is beguilingly simple and yet surprising. By placing columns at angles to the corners, he transformed a rectangular space into an octagonal one.
When he turned his attention to the church Borromini was faced (as he had been in the cloister) with the challenge of working with a very limited space. He rose magnificently to the challenge and created one of the most exciting spaces in Rome. Convex and concave surfaces alternate in a complex design. Triangles are used throughout to symbolise the Holy Trinity.
The facade, which also uses alternating convex and concave surfaces, was added after Borromini's death.