At the center of the Mausoleum of Hadrian was the Burial chamber (or Hall of the Urns) in which the emperor was buried.
The Burial chamber has three niches on the east, west and north sides, in which were the tombs of Hadrian, of his wife Sabina and probably of Aelius Caesar, the designated heir who died before them.
The Hall is now crossed along the walkway built by Valadier at the end of the 19th century, when the Helical ramp was rediscovered. Unfortunately it has become a passageway to go to the upper floors of the Castle: visitors do not understand that it was the most sacred and important room of the building.
The Hall was built with blocks of travertine and tuff and was completely covered in precious marble, of which a small fragment of pavonazzetto remains. The floor was made of white Luni marble, today's Carrara marble.
A red porphyry sarcophagus was found in the Hall of the Urns, the basin of which was reused for the tomb of the Holy Roman Emperor Otto II (973-983) and was destroyed in a fire.
Only the lid remains, which in the seventeenth century was transformed into a Baptismal font by Carlo Fontana, and today is in St. Peter's Basilica.
Even today, special illuminations occur in the Hall of the Burial Urns chamber on the days of the Summer solstice.
The Sun enters through the two window-tunnels that open into the vault of the Burial chamber, and creates rectangles of light in the three niches.
Their symbolic meaning and the thousand-year history of the Mausoleum are revealed in Marina De Franceschini's book: «Castel Sant'Angelo. Mausoleum of Hadrian. Architecture & Light», also in English language.