Borromini had to build his church on a highly limited and asymmetrical corner site. Consequently the plan of the church is indeed complicated, and to that epoch-making in the history of architecture. One could say that the basic design of the plan is elliptical, but the already existing fontain on the cut-off corner was to the effect that the architect had to omit a chapel from an earlier design. Borromini decided to create a spatial unity by incorporating the chapels into the ellipse. The shape of the site didn't admit any radiating chapels.