The church was originally commissioned by Pope Leo I (440-461), with the date confirmed by ancient coins and by dendrochronology, which places the wood used in the beams of the roof to around 455 AD, but was not consecrated until after his death. The original church had three concentric ambulatories flanked by 22 Ionic columns, surrounding the central circular space surmounted by a tambour that is high and 22 m wide). There were 22 windows in the tambour but most of them were walled up in the 15th century restoration. The central ambulatory had a diameter of , and the outer one a diameter of . Four side chapels extended from the middle ambulatory to the outer ambulatory, forming a Greek cross.