It is well known that the general form of the Colosseum is oval, and that it had four principal entrances, one of which only remains; this is said to have been the entrance for the emperor and his suite, it is not numbered as the other entrances and seats were[101]. The theatre, as we are told in the Regionary Catalogues, was calculated to hold 87,000 people, and was admirably adapted for its purpose. There were four principal staircases, by which the spectators could ascend to the highest tier of seats, and these were so arranged that the different orders could disperse without meeting each other. The numerous places of egress, called _vomitoria_, and the windows to light the staircases, were contrived with great skill. Vast as this amazing edifice still is, the whole of the outer wall with the arcades and corridors, on the south and west sides, have been destroyed, having been used as a stone quarry for building some of the largest palaces in Rome, but on the north and east sides it is tolerably perfect. A correct idea of the whole can only be formed by mounting to the top, and surveying the whole extent from thence. The finest view of the exterior is from the Thermæ of Titus, or the windows or the garden of the monastery of S. Pietro in Vincoli, on the hill opposite the north front. The great mass of the building under the corridors is of tufa, and was probably taken from that part of the second wall of Rome which passed under the south end of the Palatine close at hand; each block of tufa, being of large size and a ton in weight, was likely to be brought from the nearest point. It has been mentioned that there are piers of travertine at short intervals, as if the builders were afraid to trust the soft tufa to carry so great a weight[102]. These piers go right through the walls from top to bottom, to carry the weight of the upper gallery when full of people; tufa is too soft a material to be trusted for this purpose, and the brick facing did not add materially to the strength. It is faced with cut stone (travertine) on the exterior, and with brick on the interior. The work, as said before, was evidently carried on for a long time. Three periods may be perceived[103] in the stonework, with apparently an interval of some years between them. The upper storey is of a later date, of the time of Alexander Severus and Gordianus, and was evidently completed in great haste, of materials previously prepared, (as may be distinctly seen in the interior); this upper wall is built also in the most slovenly manner, with portions of cornices and of columns, or fragments of old tombs, built into it as mere pieces of stone[104]. On the interior of this wall large corbels remain distinctly visible, which could only be for the floor of the wooden gallery.