"All labor yields to Caesar's Amphitheater." Martial, On the Spectacles (I)
Begun by Vespasian, the Colosseum (Amphitheatrum Flavium) was dedicated by his son Titus in AD 80 with games that lasted one-hundred days, and completed by his brother Domitian, who added the substructure beneath the arena.
There were four arcaded stories, the first three each had eighty arches, framed respectively by engaged Doric, Ionic and Corinthian columns. Seventy-six of these arches were numbered to assist spectators in finding their assigned seats. On the major axis was the Porta Triumphalis, through which the gladiators paraded, and the Porta Libitinensis, named after the goddess of funerals, through which the dead and wounded were carried. On the minor axis were entrances set aside for the emperor, and for magistrates and Vestal Virgins.
An attic, which served as the fourth story, had Corinthian pilasters framing bays that alternated between windows and large decorative shields of gilded bronze that had been added by Domitian. Above them were projecting consoles that supported the masts from which the velaria or awnings were suspended. Circumscribing the amphitheater was a series of posts, one of which can be seen at the bottom of the picture.
To protect spectators from the hot Mediterrean sun, the seating area (cavea) of the Colosseum, as well as part of the arena, were screened by a huge awning. This velaria consisted of long fabric strips, connected together and suspended from ropes that hung from 160 masts set in sockets around the cornice of the amphitheater (80 addtional poles helped support the weight of the material). A special detachment (vexillatio) of marines raised and lowered the velarium like sails on a ship. To maintain tension on the lines, it has been suggested that they were tied to a corresponding number of bollards around the outside perimeter of the Colosseum. It is more probable that these posts (cippi) were used to secure barriers between the arches to facilitate management of the crowd. Five of these stone pillars still remain in place, three of which can be seen above.
The orderly seating of a potentially unruly crowd of fifty-thousand spectators was handled by a series of ambulatory corridors. They provided access to the seating (cavea), which was divided by aisles (baltei) into four successive tiers (maeniana), each reserved for a certain social class. Stairs (vomitoria) further separated these sections into wedges (cunei). Nearest the arena was the podium, which was reserved for senators and other magistrates, who were protected from the beasts by a gilded net. The equestrian order sat behind in the maenianum primum, while women were relegated to wooden seats in the upper-most tier (maenianum summum) beneath the portico. 
Beneath the wooden floor of the arena, which was covered with sand (arena) and from which its name is taken, was an elaborate substructure of service corridors, cages for wild animals, ramps, and mechanical lifts.
