Probably the best example of Borromini's genius, this church is also his last known commission - the architect returned to work on the façade some thirty years after he began the church, which he never managed to finish. The plan of the interior is a complex hybrid of an elliptical and Greek cross plan. The oval coffered dome is topped by a lantern which allows light and the Holy Spirit to descend onto the gathered worshippers. Exquisitely proportioned cloister.