The outer wall was made from about of travertine stone. This was held together by of iron clamps. There was no mortar used to hold the wall together. The outside wall has been badly damaged over the years. Large sections have fallen down after earthquakes. The north side of the outside wall is still standing. It has triangular brick wedges at each end, added in the early 1800s to hold up the wall. The rest of the outside wall that can be seen today, is in fact the original inside wall.