The altar aedicule is dwarfed by an enormous gilded bronze prothyrum, consisting of a triangular pediment supported by a pair of ribbed Composite columns on box plinths bearing relief coats-of-arms. The frieze of the pediment has an epigraph commemorating the pope on its frieze. Another pair of identical columns, set back diagonally, support two entablature lengths at the same height as the pediment entablature. The capitals and bases of these four bronze columns were bronze-founded, but the columns themselves are ancient.