The frescoes of the dome were painted on the underside of an inner brick shell. This thin and light structure was built to create a suitable surface for the painted decoration at an unknown date. The dome and the crossing is lit by eight large semicircular windows. The marble revetment and the double Corinthian pilasters on the side walls of the octagonal tambour appear Berninian, although similar double pilasters are visible on an earlier fresco in the vestibule of the Sala Sistina of the Vatican Library. On the same veduta a walkway with a balustrade is visible at the base of the tambour. This zone is emphasized by a strongly projecting stone cornice with acanthus modillions and varied rosettes.