Not many years afterwards, Constantia, the daughter of the Emperor
Constantine, suffering from a long and painful disease, for which she
found no relief, heard of the marvellous vision, and was told of
many wonderful cures that had been wrought at the tomb and by the
intercession of the youthful Saint. She determined, although a pagan,
to seek the aid of which such great things were told; and going to the
grave of Agnes at night, she prayed for relief. Falling suddenly into a
sweet sleep, the Saint appeared to her, and promised her that she should
be made well, if she would believe in the Lord Jesus Christ. She awoke,
as the story relates, full of faith, and found herself well. Moved with
gratitude, she besought her father to build a church on the spot in
honor of Saint Agnes, and in compliance with her wish, and in accordance
with his own disposition to erect suitable temples for the services of
his new faith, Constantine built the church, which a few centuries later
was rebuilt in its present form and adorned with the mosaics that still
exist.

Nearly about the same time a circular building was erected hard by the
church, designed as a mausoleum for Constantia and other members of the
imperial family. The Mausoleum of Hadrian was occupied by the bodies of
heathen emperors and empresses, and filled with heathen associations.
New tombs were needed for the bodies of those who professed to have
revolted from heathenism. The marble pillars of the Mausoleum of
Constantia were taken from more ancient and nobler buildings, its walls
were lined with mosaics, and her body was laid in a splendid sarcophagus
of porphyry. In the thirteenth century, after Constantia had been
received into the liberal community of Roman saints, her mausoleum was
consecrated as a church and dedicated to her honor. A narrow, unworn
path leads to it from the Church of St. Agnes; it has been long left