The spiral

Tucked behind this courtyard sits the strangest building of the whole walk, hidden away in the inner court of Rome's old university.

Borromini landed the commission for Sant'Ivo alla Sapienza in 1642, and for once Bernini, his great rival, was nowhere near. The floor plan comes from two crossed triangles, a six-pointed star, and that shape carries all the way from the floor up into the dome without ever shifting into anything else. There's not a single spot where the walls simply come to rest.

On top you'll find the spiral, a stone screw that winds its way upward and ends in a crown of flames. Nobody has pinned down exactly where the idea came from. Some point to the lighthouse of Alexandria, others to a ziggurat, others still to the unicorn's horn in the coat of arms of the man who paid for it. Whatever its source, it's unlike any other dome crowning in the city, and from the courtyard you can take it in all at once.

The church only opens on a handful of mornings each month, usually on Sunday, and if you catch that window you'll be standing in the most peculiar room in Rome. The courtyard around it is open on weekdays as a matter of course.

Borromini spent nearly twenty years on this. He died in 1667 by his own hand, after a long stretch of deep melancholy, and lies in the church of San Giovanni dei Fiorentini beneath a stone that carries no name.

We cross the Corso del Rinascimento toward the long, narrow square with its three fountains.