The round façade of the building was originally composed of white and pink limestone from Valpolicella, but after a major earthquake in 1117, which almost completely destroyed the structure's outer ring, except for the so-called "ala" (wing), the stone was quarried for re-use in other buildings.
Facaden var bygget af hvide og lyserøde kalksten fra Valpolicella, men efter et jordskælv i 1117, som ødelagde bygningens ydre ring bortset fra den såkaldte "ala", genbrugtes stenene herfra til andre bygninger.