In the center is the substructure beneath the arena floor. Almost forty feet deep, it contained a system of corridors for slaves, caged animals, and machinery for performances. It also held the plumbing to flood the arena for water events. The tiers of seats were strictly divided by social strata. The emperor and his family and court sat in the first tier; patricians and gentry were in the second tier; ladies were in the third tier; and common people sat on the top. Barrel vaults were the main supports of the heavy tiers.
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