In the lower series of seats the vaults under them are not original, except those on the ground-floor; and in the corridors the large corbels for wooden floors and galleries still project from the face of the wall very distinctly[105]. The mixture of stone and brick in the construction is curious, and in several parts indicates the great repairs in the fifth and sixth centuries recorded by inscriptions found during the excavations. There is a series of arches on the first floor within, some of them begun, and some completed, in stone, but the greater part have springing stones only upon the stone jambs, the arches afterwards being completed in brick, and brick vaults introduced in place of the original wooden floors and galleries; above, nearly all is brick, except the corridors and the outer facing. The construction of this part is very good throughout; the stones are in large oblong blocks, closely fitted together, originally held with iron clamps, fragments of which remain, as may be seen or felt in the interior of the building in apertures of the wall: the holes where other iron clamps have been, are left all over the face of the building, and in the corridors, always at the edges of the stones, where in rusting they have split the stone and fallen out. They were of this form [symbol][106]. On the west side, where the outer wall is gone, the inner wall (now external) shews distinctly the flat pilasters of stone carried up nearly to the top, but left unfinished, and continued with brick afterwards. Nothing is known with certainty of the architect of the work; an inscription found in the Catacomb of S. Agnes, in memory of Gaudentius, has given rise to the legend that he was the architect[107], and that he afterwards suffered as a martyr within its walls. Twelve thousand Jewish slaves are said to have been employed upon it during five years, and ten million Roman scudi expended upon it in the same time.