Santa Costanza is a 4th-century round church in Rome with well preserved original layout and mosaics. It has been built adjacent to a horseshoe-shaped church, now in ruins, which has been identified as the initial 4th-century cemeterial basilica of Saint Agnes. Santa Costanza and the old Saint Agnes were both constructed over the earlier catacombs in which Saint Agnes is believed to be buried.
According to the traditional view, Santa Costanza was built under Constantine I as a mausoleum for his daughter Constantina, later also known as Constantia or Costanza, who died in AD 354. However, more recent excavations seem to date the existing church to the time of Emperor Julian (r. 361-363), who would have built it as a funerary structure for his wife, Helena, who died in AD 360, and was herself also a daughter of Emperor Constantine.
The original structure containing the tomb of Helena might be located underneath the current church. 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. 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. The earlier triconch building of the 330s was probably indeed built for Constantina, but she later had to take second place to her sister; as Constantina's fame as a saintly figure developed in the Middle Ages, their roles became reversed in the popular mind.
The mausoleum is of circular form with an ambulatory surrounding a central dome. The fabric of Santa Costanza survives in essentially its original form. 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. The vaults of the apses and ambulatory display well preserved examples of Late Roman mosaics. A key component which is missing from the decorative scheme is the mosaic of the central dome. In the sixteenth-century, watercolours were made of this central dome so the pictorial scheme can be hypothetically reconstructed. 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.
References:The Walls of Constantinople are a series of defensive stone walls that have surrounded and protected the city of Constantinople (today Istanbul) since its founding as the new capital of the Roman Empire by Constantine the Great. With numerous additions and modifications during their history, they were the last great fortification system of antiquity, and one of the most complex and elaborate systems ever built. They were also the largest and strongest fortification in both the ancient and medieval world.
Initially built by Constantine the Great, the walls surrounded the new city on all sides, protecting it against attack from both sea and land. As the city grew, the famous double line of the Theodosian Walls was built in the 5th century. Although the other sections of the walls were less elaborate, they were, when well-manned, almost impregnable for any medieval besieger.