Hadn't known until this week how many years he spent with a chisel before he ever spent one with a pen. Born near Lugano in 1599, a stonemason's son, Francesco Castelli in those days — the Borromini came later — and packed off as a boy to the cathedral works in Milan to cut marble. Years of that, standing in stone dust. Then Rome, carving on St Peter's under Carlo Maderno, who was some sort of relation.
Go and stand in San Carlo alle Quattro Fontane sometime. It's a small church on an awkward corner where four fountains meet, and the room under the dome is an oval, not a circle. Look up. The coffering is crosses and octagons and hexagons interlocked, and each ring is smaller than the one below it, so the vault reads far deeper than it is. All the light comes from the lantern, white and flat, with a gilt sunburst tucked up inside it, and there's lettering ringing the opening with the year 1640 in it. A man who has cut stone knows what a curve costs to make.
Sant'Agnese is the other one people go for, over on Piazza Navona, twin towers above the ochre fronts and the obelisk out in the middle of the square. He didn't see either building finished as he'd drawn it. The façade at San Carlo went up after he died in 1667.
Fabien, the Discalced Trinitarians who commissioned San Carlo were an order that spent its money ransoming captives, so there wasn't much of it for building: he got the cloister and the cells up first, cheaply, and the church came after. A small budget on an awkward corner, and he still gave them that oval.
Fabien, the thing that gets me about San Carlo is that it holds hardly anyone — you're standing in a room the size of a small stage, and the ceiling does all the acting. Lantern's your only lamp, no wings, no borders, and the coffering does the perspective the way a painted flat used to.