This "Portico of the Harmonious Gods" was dedicated to the twelve Olympian deities: Jupiter and Juno, Neptune and Minerva, Mars and Venus, Apollo and Diana, Mercury and Ceres, Vulcan and Vesta. The building is a curious structure, wedged in the angle between the Clivus Capitolinus, the Temple of Vespasian, and the Tabularium. The lower level consists of seven small dark rooms whose function is unknown, possibly connected with the Treasury in the adjacent Temple of Saturn.
The upper level consisted of a platform paved with marble, with a portico of fluted Corinthian columns backed by eight small rooms (see this reconstruction and the view of the remains from the Tabularium). It is thought that gilded statues of the twelve gods and goddesses stood between the columns.
Reconstruction model courtesy of UCLA's Digital Roman Forum.
Tabularium