Even for a Roman architect, the construction of the amphitheater was a tremendous feat of engineering; and many different materials were combined to reinforce its massive fabric. Of these, travertine, a local limestone brought from quarries near Tivoli along a road that had been specially made for the purpose, evidently performed the most important function. The builders of the Colosseum seem to have adopted very much the same principles as are now employed by architects who work in steel and ferroconcrete. Using travertine blocks, they raised a skeleton framework of concentric piers and arches, then linked the travertine rings with a series of connecting lateral walls -- on the upper floors in brick and concrete, on the lower in tufa or volcanic stone; the vaults were constructed of pumice stone, which tended to reduce their weight.

Externally, apart from its statues -- no doubt a rather unfortunate addition -- the Colosseum was severely plain. Each of three lower stories formed a monumental arcade, divided by columns of a single order -- Doric, Ionic, or Corinthian. The fourth story was set with Corinthian pilasters, and a range of narrow quadrangular windows (which lighted the passage behind the gallery) pierced the wall between every other pair. Here again the material employed was travertine, and because the masons did not use mortar, they inserted iron clamps to hold the blocks in place.

The amphitheater had no less than eighty entrances; and of these seventy-six bore numbers that corresponded to numbers stamped on the spectators' tickets. Two were reserved for the emperor and his suite and led directly to the podium; two, for the gladiators themselves, who like the heroes of the modern bull ring, always entered in procession. One of the latter was a small, forbidding entry named the Porta Libitinaria -- after Libitina, the Roman goddess of death -- through which the bodies of the slain were hurried out toward an unmarked grave.

The whole area surrounding the Colosseum had been paved and railed off. From a broad approach, cobbled with slabs of lava, one entered a precinct with a pavement of travertine more than five thousand feet wide, encompassed by substantial boundary stones, a few of which remain standing. It was then an easy journey, under the numbered portal, up a broad staircase, and thence to a numbered landing and a numbered seat.

The Colosseum's anonymous designers created two distinct seating areas : an exposed double-tier of seats for the bulk of the arena's 45,000 spectators, and a series of enclosed galleries for women, slaves, and foreigners. Another of Fontana's views shows the elaborate system of guy ropes, masts, and eyes that supported the velarium, or awning, which shielded spectators from both sun and rain.

The ingenious architects of the Colosseum had carefully studied the spectator's needs. Having entered without undue jostling and crowding, he could settle down to enjoy, in comfort, a long exciting day of sport and pageantry, which began at dawn and, during the reigns of Domitian and other gladiatorial enthusiasts, lasted far into the night. Overhead stretched the protective velarium; below at the broadest extent of the ellipse, reclined the emperor and his assembled dignitaries; beyond them stretched the surface of the arena, which was often remodeled and planted with trees and rocks to resemble an exotic landscape -- a background for one of the venationes, or wild beast hunts, that frequently accompanied the gladiatorial games. Beneath the arena was a labyrinth of lightless cells that housed prisoners and beasts condemned to die, and an elaborate system of elevators that raised them as their turns came.