The smallest masterpiece

On this corner stands the church that Borromini said held his entire idea of architecture, and it came to him as his first independent commission, from an order that had no money.

San Carlo alle Quattro Fontane had to fit on the scrap of land that was left over, smaller than one of the piers of St Peter's Basilica. Borromini solved that by making the walls ripple: the facade curves inward and outward as if it were breathing, and that movement simply carries on inside.

The four fountains on the corners of the crossroads are older, from the 1580s, and they mark the point where two straight streets laid out by Pope Sixtus V meet. From here you can see an obelisk in three directions.

The church is free and closes for a couple of hours around midday. Inside, the second part of this stop is waiting for you, about the dome and the trick that makes it look taller than it is.