The Ara Pacis is made up of an enclosure which surrounds the true altar; it thus follows the pattern of a templum minus, which is described by Festo like this: "The templa minora were created by the Augurs (priests), who encircled the chosen places with wooden panels or with drapery, so that they only had one entrance, and demarcated the space with the customary formulae. The temple, therefore, is an enclosed space, consecrated in such a way that it is open on one side and has its corners clearly determined on the ground." Except for the fact that the Ara Pacis has two entrances, this description fits the monument extremely closely. Even its internal decoration shows, in the lower parts, the wooden boards, which, in archaic temples, delimited the space that had been consecrated with sacred formulae.
The interior of the enclosure, like the exterior, is divided into two registers placed one above the other and separated by a band of palmettes. In the lower register the decoration seems to use as a motif a simplified version of the planks from the wooden enclosures which marked out the sacred space; the higher register, however, is richly adorned with festoons and bucranes (animal skulls), interspersed with paterae and ritual cups. These motifs also are related to the decoration that was placed above the wooden enclosure, in this case garlands heavily weighed down with ears of corn, with berries and fruits of every season, wild and cultivated, attached to the supports by means of ribbons and sacred fillets.