According to the traditional view, the mausoleum was built in the reign of Constantine I for his daughter Constantina, later also known as Constantia or Costanza, who died in 354.

Despite the loss of the coloured stone veneers of the walls, some damage to the mosaics and incorrect restoration, the building stands in excellent condition as a prime example of Early Christian art and architecture.

[5] The large porphyry sarcophagus of either Constantina or her sister Helena has survived intact, and is now in the Vatican Museum – an object of great significance to the study of the art of Late Antiquity.

[5][1] Santa Costanza is located a minute's walk to the side of the Via Nomentana, the road that follows the ancient Roman route running north-east from Rome to Nomentum or Mentana.

The underground catacombs had been built there many years earlier and contained the relics of Saint Agnes who was martyred as a thirteen-year-old,[2] and over whose tomb was a small chapel.

[6] The funerary hall or "Constantinian basilica" was built first from 338 as a result of Constantina's devotion to Saint Agnes, a legend later considerably elaborated, but early Christians believed that their souls benefited from being buried close to martyrs, which was almost certainly a major attraction of the funerary hall to those who paid to be buried in it.

[8] Other basilicas with floorplans typical of the Constantine era are San Sebastiano fuori le mura on Via Appia, San Lorenzo fuori le mura on Via Tiburtina and the anonymous basilicas on Via Prenestina and Via Ardeatina,[9] the latter recently discovered and attributed as the burial place of Pope Mark (d.336).

[12] The structure of Santa Costanza reflects its original function as the mausoleum of one or both Constantine's two daughters, Constantia and Helena, rather than as the church it became much later.

It is built of brick-faced concrete and its structure is basically two rings supported by columns placed around a vertical central axis.

Opposite the entrance in this central space there is "a kind of baldacchino...rises above a porphyry plaque which, below the middle arch of the center room, once seems to have carried the princess's sarcophagus".

As evidenced by surviving Renaissance-era illustrations the church was likely once covered with mosaic decoration but today all that remains are those in the two apses and those in the ring barrel vault.

[20][21] In the ambulatory wall there are two shallow apses, each with a mosaic showing Christ as the Pantocrator, the earliest surviving examples of this depiction; they probably date to the 5th or 7th century, though there has been much discussion of this.

[12] Like many mosaics of the period, both have suffered from restoration and both show elements of Roman imperial imagery, representing early examples of the conflation of this with Christian art.

This concept and picture of Christ as the almighty ruler and creator of the world would be the norm in the artwork of later churches, but it first appears here at Santa Costanza.

[23] The 4th-century mosaics on the ambulatory vault are contemporary with the building, and show a stark contrast to those in the apses, being essentially secular in appearance and make no obvious Christian allusions.

The surface of area of many of these mosaic panels is occupied by a profusion of vine and plant ornament as well as containing geometric patterns, small heads or figures within compartmented frames, birds with branches of foliage, vases and other objects.

[24][21][25] The theme continues in the floor mosaics which were similar in style to those in the ambulatory, filled with cupids, birds, and Bacchus and grapevines.

The mosaics of the central dome no longer exist, but a picture of them can still be reconstructed as between 1538 and 1540 Francisco de Holanda made watercolour copies of what then survived.

In these several biblical scenes appear, resembling catacomb paintings from the 3rd century, including Susanna and the Elders, Tobias, the sacrifice of Cain and Abel, the sacrifice of Elias on Mount Carmel, possibly Lot receiving the angels, Moses striking the rock for water, and possibly even Noah building the ark.

[27] Constantina's sarcophagus has complex symbolic designs in relief: "the surface is dominated by an intricate pattern of stylized vine-stems into which are fitted cherubs...with this scene of Dionysiac exuberance, and the hope of future blessedness which it implies, two peacocks, birds of immortality, are completely in accord".

[30] The veneration of Constantina as "Santa Costanza" (Saint Constance) is only known from the 16th century onward, and her name is not included in the Roman Martyrology.

[1] That could suggest that the current church is the second Christian building on the site, and may be some decades later than traditionally thought, being built as a mausoleum for Constantina's sister Helena in the reign of her husband Julian the Apostate.

If true, the larger of the two porphyry sarcophagi there would belong to Helena, and the smaller to Constantina, the opposite of what has been traditionally thought.