The octagonal dome sits on a high drum with a large rectangular window on each side. In between the windows are conjoined pairs of Corinthian pilasters supporting a cog-wheel entablature. The lantern is Carlo Rainaldi's design, and involves eight engaged Corinthian columns with narrow round-headed windows in between. The columns support another cog-wheel entablature, decorated with eight flaming torch finials and with an ogee cupola on top. The whole ensemble is topped off by a cross on a globe.
On the apex of the pediment is the coat-of-arms of Pope Innocent X