The worst destruction was not caused by the invasions and looting of the barbarians, who certainly made every precious object disappear.
It was made by the Romans themselves, simply because they needed ready-to-use building materials and did not hesitate to demolish buildings, remove bricks, dismantle marble and columns to reuse them elsewhere.
And since during the Middle Ages metals were scarce, columns and trabeations were dismantled or drilled to recover the metals that held them together, especially lead and bronze.
The Pantheon also suffered a series of looting and thefts of materials, but the beauty and majesty of its dome saved it from destruction. Also thanks to Emperor Phocas, who in 609 AD, agreed to donate «the temple called Pantheon» to Pope Boniface IV who had asked for it.
The Pantheon thus became the church of Santa Maria ad Martyres, and remained almost intact. Nonetheless, it suffered a series of thefts of metals, that during the Middle Ages were considered much more precious than marble or sculptures.
In 603 AD, Emperor Constantius II had the gilded bronze tiles covering the roof of the Pantheon's dome removed, causing dangerous water infiltrations that were remedied by Pope Gregory III (731-741) who had new lead tiles installed.
Officially, he said he had done it to have the bronze needed for Bernini's baldachin in St. Peter's Basilica. But in reality that bronze was used to make the new cannons of Castel Sant'Angelo, the Mausoleum of Hadrian then transformed into a fortress.
Finally, in the seventeenth and eighteenth centuries, pretending to clean and restore them, many of the precious marbles that covered the walls were stolen and to disappeared - especially the red porphyry - replacing them with trompe l'oeil frescoes painted to look like marble.
They are so realistic that many believe that they are still the original marbles, but this is not the case.
Only the great columns in giallo antico marble were left in place, because otherwise the building would have collapsed.
In reality the colors ofthe wallss were completely changed, going from a prevalence of white and red to the current green and yellow.
You can read and discover these and other events of the history of the Pantheon in the Middle Ages and the Renaissance in the books by Marina De Franceschini: «Pantheon. Architecture & Light» and «Castel Sant'angelo. Mausoleum of Hadrian. Architecture & Light», both in English edition. (on sale in the Bookshop of Castel Sant'Angelo National Museum in Rome) .