The geometrical plan is that of a coved arc inserted into a flat plane, and according to architectural historians Borromini, the first architect, took the idea from the Villa of Hadrian at Tivoli. The façade has six gigantic Doric pilasters rising to the roofline, the outer pair of which contain the coved curve only in the lower half. The upper half of the façade has this curve confined to the inner pair of pilasters, with the walls between the inner and outer pilasters sweeping back.