The Amphitheater’s loss of its original function led over the centuries to the transformation and decay of its structures. At first several dwellings were established in the ground-floor ringfacing and everything that could be reutilized in some way, including the metal clamps that in antiquity held the blocks of stone together. In order to remove them holes still visible were made in the masonry.
The plunder gradually turned several monumental buildings under construction at that time, especially St. Peter’s. The Church subsequently turned the arena into a holy place: from the early sixteenth century it housed a chapel and from 1720, along its perimeter, the Stations of the corridors. Later, in the twelfth century, the building was incorporated into the Frangipane family’s fortress and remained part of it until the first half of the thirteenth century. Meanwhile, systematic plunder had already begun of the blocks of travertine, the marblesections of the monument into real quarries and even led to the demolition of the south exterior ring.
A stricter conservation policy was urged in vain by Roman humanists in the fifteenth century. Indeed, excavation activity actually increased to provide materials for the Cross. In the seventeenth and eighteenth centuries the demolitions slacked off and the first, half-hearted conservation measures were adopted. During the nineteenth century the first systematic excavations were undertaken by Carlo Fea (between 1812 and 1815) and Pietro Rossi (in 1874-1875). The excavations brought to light again the subterranean structures of the arena, thus making it necessary to remove the shrines and the chapel, which was originally located in the east section of the cavea. During the same period the first significant reinforcement and restoration work was carried out.
Between 1805 and 1807 Raffaele Stern built the brick abutment in the east section, while in 1827 Giuseppe Valadier restored the wall of the same exterior ring on the opposite side. Finally, the work carried out by G. Salvi and L. Canina between 1831 and 1852 regarded the interior structure in the south and north sections. Further restoration, especially in the cavea and the cellars, took place in the 1930s.