Colosseum - Wikipedia

The outer wall is estimated to have required over 100,000 cubic metres (3.5 million cubic feet) of travertine stone which were set without mortar; they were held together by 300 tons of iron clamps.

However, it has suffered extensive damage over the centuries, with large segments having collapsed following earthquakes. The north side of the perimeter wall is still standing; the distinctive triangular brick wedges at each end are modern additions, having been constructed in the early 19th century to shore up the wall. The remainder of the present-day exterior of the Colosseum is in fact the original interior wall.