On the sides flanking the door, there is the outline of a blocked door with an arc lintel, over which is the outline of a blocked round-headed window. These features are thought to be Constantinian. Each side now has an extant 17th century rectangular window, which is set in a larger partly blocked round-headed window which is 5th century. The roofline frieze bearing the heraldic emblems of Pope Alexander VII is by Borromini. The low octagonal 16th century dome has a circular window (oculus) in each side and a cap with eight tiled sectors.