He built his Forum because the two already in existence could not deal with the recent great increase in the number of lawsuits caused by a corresponding increase in population; this was why he hurriedly opened it even before the Temple of Mars had been completed. Public prosecutions and the casting of lots for jury service took place only in this Forum. Augustus had vowed to build the Temple of Mars during the Philippi campaign of vengeance against his father Caesar’s assassins. He therefore decreed that the Senate should meet here whenever declarations of war or claims for triumphs were considered, and that this should be both the starting point for military governors, when escorted to their provinces, and the repository of all triumphal tokens when they returned victorious. (Suetonius, Aug. 29)
While little remains of the forum and the Temple of Mars Ultor, the high podium, column bases, retaining wall and outline of the temple pediment give a clear indication as to the imposing size of the complex.
The entire complex was set against a gigantic wall made of blocks of peperino, gabina stone and travertine, up to 33 meters high in the highest part. This was to protect the forum from the subura which lay behind the wall where the wooden buildings of an overcrowded neighbourhood posed a significant fire risk; peperino and pietra gabina were considered fireproof. The two entrances to the Forum from the subura can still be seen (pic below), with a triple arch to the left of the temple and a monumental arch in travertine to the right.
The steps and much of the facade were made from Carrara marble
This artist drawing shows the forum with the Temple as its centre-piece. The maidens on the upper storey of the colonnades were taken from a cast which was made by Augustus' engineers from one of the caryatids supporting the erechtheion on the north side of the Acropolis which Augustus had ordered to be repaired during a visit to Athens.
Between the caryatids were a series of shields, depicted thus to represent Alexander the Great's custom of placing his shield in a temple following a military victory. Fragments and reconstruction can be seen in the Museo dei Fori Imperiali in Trajan's Market
In order to make his forum bigger than Julius Caesar's, Augustus added two hemicycles, or exydrae, and these provided extra space for sculptures of the summi viri (great men) of Rome's history, specifically statues of the divine along with the most illustrious members of the gens Iulia (Augustus' family). Trials would be held inside the hemicycles by the ancient Roman judges (praetores). In the northern hemicycle the praetor urbanus would preside over lawsuits involving Roman citizens while in the opposite hemicycle the praetor peregrinus would take care of legal matters involving foreigners who did not have the Roman citizenship.
The northern portico ended in a lavishly decorated room, the so-called 'Hall of the Colossus'. (the outline of the roof apex can still be seen on the retaining wall to the left of the temple apex). A colossal statue of Augustus (between 11 and 12 meters tall) was placed on a base in this hall as a symbol of the equality of justice administered by the judges in the nearby tribunals.
Further deference to to ancient Greece can be seen in the use of zoomorphic capitals within the forum, some of which are beautifully preserved in the Imperial Forum Museum. The capital below shows a clearly defined pegasus.
The Pediment of the Temple
The pediment of the Temple was used to reinforce Augustus' divine origins. It centres around a statue of Mars brandishing a spear as a reference to war and a sheathed sword as a reference to the resulting peace. He is seen with his foot standing on a globe, clear allusion to Rome's dominion over the world. It is highly likely that Augustus wanted people to recognise a link between himself and Mars with the princeps being the avenger of Caesar's death and the one who brought peace to the empire.
To the right of Mars we see Venus as Venus Genetrix (the mother), depicted with a Cupid on her shoulder. This not only aligns Augustus with the divine goddess but also with Julius Caesar whose temple in his own forum was dedicated to the deity. Next to Venus were Romulus and the personification of the Palatine. To the left of Mars were the statues of Fortuna, the Goddess Roma and the personification of the Tiber.
Two figures of winged Victories in gilded bronze stood out on the lateral corners of the pediment and the foot from one of them was found in the 1930s:
