A kind of theater "in the round," a round or oval structure having a central stage or arena surrounded by tiers of seats rising gradually outward from it.Example: Roman, Rome, Colosseum (Flavian Amphitheater), 70-82 CE. A vast ellipse in plan, length 620 feet x width 513 feet (188 x 156 m), with eighty external arcaded openings on each storey, those on the ground floor forming entrances from which the various tiers of seats were reached. see thumbnail to rightSee a cutaway view. The upper storey was added 222-224, around the top of which were sockets for 240 wooden masts which carried a massive canopy (velarium). The fa?ade's entire height is 157 feet 6 inches (48.5 m). Each of the four storeys are pierced by arches, and have attached three-quarter columns and entablatures, Doric in the first story, Ionic in the second, Corinthian in the third, and above these are pilasters of Corinthian design, with small square window openings in alternate bays. The wooden floor of the arena is an oval 287 x 180 feet, surrounded by a wall 15 feet high, behind which were the podium, with the imperial throne, and seats for the pontifex maximus, vestal virgins, senators, praetors, and other officers of the state. Behind the podium rose the auditorium seats for some 50,000-75,000 spectators, with corridors and stairs beneath, while dens for wild beasts were under the lowest tier, on a level with the arena. The seats (now removed) were in four main horizontal divisions. The construction of this amphitheater is notable for its combination of materials, carefully chosen for the purposes to which they were applied. The design of this structure was made possibly because of the Romans' invention of concrete, used primarily for the foundations. Different types of concrete were utilized depending on their need to be especially solid, or to be reduced in weight. Travertine was used to face the exterior, for the piers and arcades, tufa infill between piers for the walls of the lower two levels, brick-faced concrete for the upper levels and for most of the vaults, limestone largely for floors, and marble for seats, columns, and ornament. Even in its current ruined state, the Colosseum recalls the gladiatorial contests, the naval displays, and the martyrdom of Christians which took place within its walls before it became a medieval fortress or was plundered to provide building materials for Renaissance palaces and churches. Another view of the Colosseum.