The Ara Pacis basically consists in a precinct that encloses the mensa, the sacrificial proper altar, where sacrifice was performed. Therefore the Ara takes up the shape of a templum minus, according to the description handed down by Festos: "The templa minora are created by the augures by fencing a chosen area with wooden planks or with cloth, so that there is only one entrance, and delimiting the area with established formulae. Thus the temple is a fenced and consecrated place with an opening on one side and corner-posts well set in the ground" (Lindsay 1913). With the exception of the entrance, that in our case is double, on the east and the west sides, such definition suits the Ara Pacis particularly well and accounts not only for the precinct framed by its four angular pillars, but also for its interior decoration, unanimously recognized, in its lower part, as a reproduction of the wooden fence that delimited the enclosure that was "inaugurated" with sacred formulae.
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The precinct rests on an almost square marble base, measuring 11.630 meters on its open fronts and 10.625 meters on its northern and southern sides. In 1938, when the excavations were completed, it was decided to leave part of the monument under the Palazzo Fiano, except for some large slabs which, as an example, were taken and reinserted in the western side, under the panel with the Lupercal, and in correspondence of the north processional frieze. The remaining parts were recreated, according to the original shape and size, in Carrara marble. Above the base, the precinct raises up to ca. 6.3 metres. Also its external side is divided in two zones or decorative registers: the lower with acanthus scrolls and the upper with figured reliefs. The registers are separated by a band, 33 cm high, with a meander, or more precisely a swastika motif, on the external side and with a palmette motif on the internal side. This band, that was largely reconstructed on the basis of the few original fragments rescued during the 1903 and 1937-38 excavations, runs along the four sides of the altar, interrupted only by the four pilasters that mark the corners of the precinct. The band’s monumental size suggests that it has some semantic value, as evidenced by the palmette motif, that is often symbolically associated with the celebration of victories. As to the meander, an ornamentation that began to be widespread in the Augustan period, it has been noticed that this motif is frequently associated with mythical scenes related to a Trojan context; therefore, they might represent a conscious reference, "continuous and aware at least until the early imperial age," to the origins of Rome (Polito 2002). This, however, does not exclude the earlier symbolical meaning of the swastika, that in Troy—where the motif was widespread, as modern excavations since Schliemann have evidenced—as well as in other oriental, or even Etrusco-Italic, religious contexts, was the symbol of the sun and of the cycle of its vital energy. Such connection is therefore very significant both in consideration of the forms of the Augustan propaganda, and also within the more limited context of the Ara Pacis, where the meander is used to demarcate the level of the representation of nature, meant as plants and animals, from the human and mythical universe depicted above. The upper level of the precinct presents, along the southern and northern sides, a parade of historical figures and in the four panels to the sides of the openings as many mythical scenes. The position of the panels appears significant. In fact who approached the monument toward its western façade would be facing two of the most topical moments of Rome’s prehistory: to the right Aeneas performing a sacrifice under the eyes of the Penates, the ancestors from Troy, to the left the finding of Romulus and Remus under the eyes of Mars, on their behalf the ancestor of all Romans. Who, instead, approached the monument toward the east façade, or going down the Via Flaminia, would find himself in front of the simultaneous representation of Victorious Rome and of Tellus, that symbolized the happiness of the present Golden Age. The same subject distribution allows further specification: from the north side, towards the town’s outer edge, one could admire the local ancient roots and the warlike virtus handed down by the myth (Mars-Rome); from the south, towards the town’s inner side, the Trojan origins and the virtues exerted in lime of peace (Aeneas-Tellus) are presented. The proper altar, on which animals were offered and wine was libated, occupies almost all the precinct’s enclosure, from which it is separated by an aisle slightly wider than a meter. It has to be mentioned that the floor of this room was tilted slightly outwards, in order to facilitate the drainage both of rain and of the water used for the washing after the sacrifices, through twelve gutters that opened along the perimeter of the podium. The architecture of the internal altar is quite complex. Its lower part, about 1.20 metres high, is made of a four-stepped podium with a truncated pyramid shape. On the podium, a metre higher, stands a second base with four more steps on the west façade. On the level reached by the eighth step stood the mensa, enclosed between two lateral avant-corps. The priest would enter the altar climbing the ten steps of the external (light of steps that went up from the Campus Martius, then, once in the precinct, would climb eight more steps to reach the sacrificial table, 3.25 metres higher than the aisle. Except for the stepped structures, the whole internal altar of course must have been lavishly decorated with reliefs, but, since one of the foundations of the Palazzo Fiano rested just on its centre, very little has survived as a testimony of such ancient magnificence. The scarce evidence that was almost entirely rescued during the excavations of 1937-38, is represented by two beautiful crowning slabs, with acroteria with vegetal volutes and winged lions, that were placed on top of the short sides of the sacrificial table. In their lower part these slabs are decorated with a frieze, partially preserved, representing a sacrifice; perhaps as it has been repeatedly suggested, the one tributed to the Pax Augusta established by the Senate, and that it had to be celebrated every year, on the thirtieth of January, the anniversary of the altar’s consecratio.