The Mausoleum of Hadrian had a monumental entrance Vestibule from which a long spiral Ramp went to the upper floor and reached the Burial hall passing through a second Vestibule.
In medieval times, when the Mausoleum was transformed into a fortress, that ramp turned out to be a direct and indefensible access route to the upper part of the Castle.
At the end of the fourteenth century Pope Boniface IX therefore decided to bury it and wall it up, and opened a new access on a higher level, equipped with a drawbridge.
Thus the memory of the existence of that long corridor was completely lost; it was rediscovered in 1825 by Luigi Bavari, a major of the Papal Engineers, during the restoration work on the Castle.
The discovery completely changed previous knowledge about the Roman structures of the Mausoleum.
The original access built by Hadrian, with the lower Vestibule, the spiral ramp and the upper Vestibule which led to the Burial hall, was thus restored.
You can read this and much more about the millenary history of the Msueoleum in in Marina De Franceschini's new book
«Castel Sant'Angelo. Mausoleo di Adriano. Architettura e Luce»- see the preview on the side.