It is a bulding with a central octagonal plan composed of two bodies: an internal higher one that contains the cupola, and an external one that contains che perimetral ambulatory on the ground and the women's gallery on the first floor. The planner knew how to organize the architectural space with a wise game of full and empty, of curved and plain surface giving place to altering lights and shadows, which together with the colour effects of the marble, of the frescos and, excially of the mosaics, gives to the whole an extraordinary lightness.