The intention of this edifice was to eclipse the original St Peter's in Rome, however the then Pope decided this was not such a good idea, even though it was dedicated to the patron saint of the city, and canned the completion not long after 1392. Inside you can watch the sundial laid out on the apse floor, timed by the ray of sunlight falling through the roof. The admission is free and actually offers free guided tours on Tuesdays, Thursdays, and Saturdays at 11.30am.