The Colosseum could accommodate approximately fifty thousand people seated, but with standing room it could reach eighty thousand. Since it was built on the artificial lake created by Nero for his Domus Aurea, the building needed particularly solid and robust foundations, which were made of tuff and travertine.
The enormous travertine blocks measured almost three meters and were joined with metal T-clamps, which were plundered during the Middle Ages, when metals were needed. The holes made to remove the metal can still be seen in the blocks.
In the building's basement, the foundations can still be seen, made with travertine blocks from the Tivoli quarries.
They were transported on large barges along the Aniene and Tiber rivers, and from the port of Ripetta via a specially built road. Centuries later, the same would be done with the Tivoli travertine blocks which were used to build St. Peter's Basilica.
Seen from the outside, the Colosseum has four floors: the first three have arches framed by semi-columns with Tuscan, Ionic, and Corinthian capitals; their walls were thinner on top, to reduce weight.
The fourth floor is made of masonry with few windows, and the cornice at the top has supports with holes for the poles that supported the velarium, a huge canvas made with a series of sails that sheltered spectators from the sun.
One hundred sailors, housed in a special quarters not far from the Colosseum, were in charge for its assembly and dismantling.
Seating was strictly divided according to social class. The lowest row, in front of the cavea, had marble seats for the senators. To protect them from wild beasts, there was a fence with elephant tusks protruding toward the arena. Archers were stationed inside to intervene in case of danger.
Then there were the other series of seats, divided into sectors, reserved for the other social classes.
The first level (maenianum primum) had fourteen steps made of bricks and covered in marble: it was reserved for members of the equestrian order.
The lower classes occupied the second level (maenianum secundum), while plebeians and women sat in what today would be the theater gallery, the summa cavea, the top flor, seated on wooden benches or standing.
Access to the various sectors followed the same system as today's stadiums. There were 76 numbered entrances, and each spectator had a ticket indicating the number of the arch.
Corresponding to the amphitheater's four main axes there were four special and reserved entrances: one led to the imperial box, the pulvinar, on the south side, where the emperor could observe the spectacles. The other three entrances were reserved for important figures such as magistrates, members of religious orders, and the Vestal Virgins.