The Mausoleum of St. Helena, attached to the Basilica of Mausoleum of Sts. Marcellinus and Peter were located on the Via Labicana on a large estate owned by Helena, Constantine’s mother by around 325. The basilica was built beside the tombs of Sts. Marcellinus and Peter. Its apse was orientated towards the west, while on its east end was a rectangular hall that led towards a domed rotunda that served as Helena’s mausoleum. The mausoleum is also known as Tor Pignattara (“Tower of Vases”), because of the amphorae inserted in the walls. It was built on a large estate owned by Helena near her villa. 
The mausoleum of St. Helena was a domed mausoleum, following other mausolea built by the Constantinian dynasty and the Tetrarchs. It was around 28 meters in diameter on its exterior, with niches placed on its interior walls, it has a tall drum with large windows that supported a dome. The dome is mostly gone now, but it and the walls of the interior may have been decorated with mosaics and colored marbles. As Helena and Constantina had large, elaborately decorated porphyry sarcophagi placed in domed mausolea, it seems likely that Helena’s mausoleum would have also been decorated with mosaics as found in Constantina’s mausoleum (which is probably the surviving Santa Costanza). The niche opposite the door probably housed the sarcophagus holding the remains of Helena. The large mausoleum is one of several late Roman imperial mausolea to be built in the form of a domed rotunda, though it was the first to be attached to a church. While it has been argued that Constantine once considered using the mausoleum as his own, certainly by the time it was completed he had already decided to make Constantinople his capital when it was almost completed. 
Between the Via Labicana and the basilica was a porticoed courtyard that gave access to the basilica. The church ran parallel to the road, and its entrance was on its south side (one of its long sides). A narthex separated it from the circular domed mausoleum that abutted the east end of the basilica. The basilica, with wide aisles separated from the nave by rectangular piers, had the characteristic rounded west end with ambulatory. On the north side was another walled enclosure, though without porticoes for additional burials. 
The monumental red porphyry sarcophagus found at the mausoleum is believed to have held the remains of St. Helena, who died around 335. While there is a tradition that Helena’s body was buried or later moved to Constantinople, the evidence points to this at least being her original resting place. On the lid of the sarcophagus figures of cupids and victories hold garlands, while on the very top there are two lions either side of the ridge - one sleeping, the other lying down. When news of Constantine, death reached Rome, the Senate apparently requested that the emperor, body be transferred to Rome. Perhaps Constantine himself may at one time have planned his burial there.  Since the subject matter seems unfitting for imperial lady, leading scholars to suppose that the sarcophagus was originally made for a male member of the Imperial family, such as Helena's husband, Constantius Chlorus or, more probably, Constantine himself. Regardless, it is clear that Constantine was buried in Constantinople. In 1777 the sarcophagus was brought into the Vatican and restored by Gaspare Sibilla and Giovanni Pierantoni and mounted on four lions carved by Francesco Antonio Franzoni. The coffin is carved with military scenes with Roman soldiers on horseback and barbarian prisoners.