The altar enclosure is set on a wide marble base, which is almost entirely restoration work. The enclosure is divided into two decorative registers: the lower is decorated with floral motifs, the upper with figures, mythological scenes around the two entrances and a procession of recognisable individuals from the period on the other two sides. These two registers are separated by a band of swastikas. This too is largely reconstructed.
Upper register. West side.
On the front of the enclosure to the left hand side, a panel survives which depicts the myth of the foundation of Rome: Romulus and Remus are shown being suckled by the wolf, watched by Faustolo, the farmer who adopted and brought up the twins and Mars, the god who begot them on the Vestal priestess Rea Silvia. The fig-tree (ficus ruminalis), under which the twins were suckled, is shown at the centre of the composition. The talons of a bird can be made out in the tree: in 1938 these were sketched in as those of an eagle; however they could alternatively belong to a woodpecker, which, like the wolf, is sacred to Mars. The god is shown dressed for war and carrying a spear; his helmet is decorated with a griffin and his cuirass with the head of a Gorgon.
On the right of the enclosure's facade a relief shows Aeneas, already well on in years, sacrificing to the Penates. He is therefore dressed in the clothes of a priest, with his head covered, as he makes an offering at a rustic altar. The lower part of his right arm has been lost, but it almost certainly held a patera, a type of ritual cup. The idea is supported by the presence of a young acolyte in the rite ( camillus) who is carrying a tray of fruit and bread and, in his right hand, a jug. A second acolyte is driving a sow to the sacrifice. The scene is probably taking place where the city of Lavinium was founded, at least if it is interpreted in the light of Book VIII of the Aeneid. It has recently been hypothesised, however, that the figure making the sacrifice is Numa Pompilio, the second of the seven kings, who celebrated peace between the Romans and the Sabines in the Field of Mars itself by sacrificing a sow.
On the east side of the enclosure, to the left is a panel depicting Tellus, Mother Earth, or, according to a different interpretation, Venus, Aeneas' divine mother and the founder of the Julian family, of which Augustus himself was a member. Yet another reading interprets this central figure as the Pax Augusta, the Peace from which the Altar takes its name. The goddess sits on a rock, dressed in a light chiton. Her veiled head supports a crown of fruit and flowers, at her feet are an ox and a sheep. Two putti hover at the goddess' side; her gaze is turned towards one of them, holding out an apple. In her lap, a cluster of grapes and pomegranates finishes off the portrait of a fertility goddess, responsible for the flourishing of men, animals and plants. At the sides of the panel two young women are depicted, the sail-making winds, one sitting on a sea dragon, the other on a swan, symbols of the gentle winds of the sea and earth respectively.
On the right panel a fragment of a relief of the goddess Roma survives, whose figure has been completed by sketching in on the mortar. As she is seated on a trophy of weapons, this can only be the goddess Roma. Her presence on the altar should be read in close conjunction with that of the Venus- Tellus figure, since prosperity and peace are guaranteed by Roma Victorious. The goddess is represented as an Amazon: on her head she wears a helmet and her right breast is bared, a baldrick hangs across her shoulder, and in it a short sword, in her right hand she holds a spear. Honour and Virtue were very probably also depicted in the scene, positioned on either side of the goddess, in the form of two young masculine divinities.
On the north and south sides, two dense crowds of people are shown, moving from left to right; amongst them priests, acolytes of the cult, magistrates, men, women and children, whose historical identity can only be constructed hypothetically. It is not entirely clear what the procession is doing: some think that the scene shows Augustus' return, specifically the ceremony of welcome paid to him at his return from his long absence in Gaul and Spain; others think it represents the inauguration of the Ara Pacis itself, the ceremony of 13 B.C. during which the space on which the altar would be built was defined and consecrated.
The procession begins, on both sides of the enclosure, with the lictors, followed by members of the great priestly schools and perhaps by the consuls. Immediately after this members of Augustus' family start to appear. On the South side Augustus himself has been securely identified, crowned with laurel, as well as the four flamines maiores, priests with a characteristic headgear topped with a metal point. Agrippa appears next, shown with his head covered by the edge of his toga and with a roll of parchment in his right hand, and finally his son, Gaius Caesar, shown as a child holding onto his father's clothes. Agrippa was Augustus' right-hand man, his friend and also his son-in-law, being his daughter Julia's second husband. He was also the father of Gaius and Lucius Caesar, Augustus' grandchildren, whom had been adopted by their grandfather with the intention that they should succeed him.
Gaius is turning towards the female figure who follows him, who is usually identified as Livia, Augustus' wife. She is depicted veiled and crowned with laurel, which marks her as of high rank. A more recent interpretation, however, suggests that she should be identified as Julia, following behind her husband and her first born.
Tiberius is usually identified among the male figures who follow; however there is some doubt as to this interpretation as the figure in question is wearing plebian shoes, which is particularly inappropriate for Tiberius, who was a member of one of the most ancient noble families of Rome.
This figure, whether Tiberius or not, is followed by a family group, probably made up of Antonia the Younger, Augustus' niece, her husband Drusus and their infant son Germanicus. Drusus is the only figure to be shown in military dress - he wears the characteristic paludamentum: in 13B.C. he was occupied in fighting the Germanic tribes to the East of the Rhine. A second family group follows, made up in a similar fashion by Antonia the Elder, another of Augustus' nieces, her husband Lucius Domitian Enobarbus, who was consul in 16 B.C., and by their children Domitia and Gneus Domitian Enobarbus, the future father of Nero.
Reading from the left, among the lined up figures Lucius Caesar can first be identified, Agrippa and Julia's second child, who also had been adopted by Augustus. Here he is shown as a very young child, being led by the hand. The veiled female figure who comes after him could be his mother Julia, certainly the gazes of everyone around converge on her. Many people maintain, however, that Julia should be identified on the other side of the procession, in the place of Livia, who would then be substituted on this side. The matronly figure behind Julia/Livia is generally identified as Octavia the Younger, Augustus' sister. Between the two women the figure of a young man stands out in lower relief, identified as Agrippa's third son by his first wife, Marcella the Older. Behind Octavia the small figure of Julia the Younger is clearly visible. She, as Augustus' granddaughter, enjoyed the right to appear first among the children present at the ceremony.
The identity of the figure behind the infant Julia remains extremely uncertain.
Lower register. North and south sides
The lower register of the enclosure is decorated with a floral frieze, composed of spirals which start from an exuberant burst of acanthus foliage; from the centre of the acanthus a floral candelabra rises vertically. From the acanthus' spirals ivy leaves grow, as well as laurel and vines, tendrils and palmettes branch off, and while the stalks taper away, curling into spirals, flowers of every type blossom forth. In the depths of the vegetation small animals find a home, as well as twenty swans with outstretched wings, which articulate the rhythm of the composition. This floral relief is often related to Virigil's fourth Eclogue, in which the Golden Age, the return of peaceful, happy times is announced by the copious and spontaneous production of fruit and harvests. As well as a generic reference to fertility and abundance, consequent on a return to the Golden Age, the frieze can also be read as an image of the peace of the gods, of the reconciliation of the divine forces which reign over the entire universe, made possible by the coming of Augustus.