The original structure must have been very particular: under the reigns of Vespasian and Domitian, inside the Colosseum, also the naumachia, that is of the naval battles. This may mean that originally the flooring of the Colosseum had to be such as to allow a large flow of water, perhaps maintaining the original pond created for Nero's house.

The large foundation pit, in the shape of an elliptical crown, was thirteen meters thick and inside the Roman engineers dug a series of channels for the outflow of rainwater. Once the elevation of the monument was built, the surrounding areas were buried until the current level of the square was reached, which was then paved. The planks of the vast area of ??the arena rested on a series of parallel walls, in which the housings of the elevators were obtained that were used to transport the beasts and the gladiators.
In the basement of the Colosseum, illuminated by torches and oil lamps , was accessed through four corridors placed along the axes of the building, so that it was possible to reach these rooms even with wagons. The crypto-portico to the north connected the Colosseum basement with the Ludus Magnus (the nearby gladiator barracks), where there was a series of residential buildings that opened onto a courtyard with an elliptical outdoor gymnasium, a miniature amphitheater. From the basement, near the stage where the emperor attended the shows, some ladders rise, which allowed direct connection with the imperial stage. From this part another underground corridor, illuminated by skylights, which originally had a marble cladding and the vault decorated with painted stucco, was used by the emperors to access the Colosseum without being seen.
Inside the cavea with the steps for spectator seats was divided into five horizontal sectors (maeniana), reserved for different categories of public: the lower sector, reserved for senators and their families, had wide and low steps that housed wooden seats (subsellia); followed by the maenianum primum, with eight marble steps, the maenianum secundum, divided into imum (lower) and summum (upper), again with marble steps, and finally the maenianum summum, with about eleven wooden steps inside the portico crowned the cavea (porticus in summa cavea): the architectural remains of the latter belong to the remakes of the Severian or Gordian II period.
The different sectors were separated by high podiums (precinctio), in which the access doors (vomitoria) opened, protected by marble barriers (dating back to the 2nd century AD restorations. The names of the senators to whom the lower seats were reserved were inscribed on the balustrade.
The spectators reached their seats by entering from the arches reserved for them. Each of the 74 arches for the public was distinguished by a numeral, engraved on the keystone , to allow spectators to quickly reach their seats.

Over the years, fires, earthquakes and man inflicted blows rather heavy at the Colosseum:
In 217 AD a fire destroyed the upper structures; the building was renovated by Elagabalus and Alexander Severus, and was reopened in 222 without the work being completed. There were other fires in the 250/252 and in 320 which caused damage to the Colosseum and there were other reconstructions under Constantine and King Odoacer (Odoacer, first of the barbarian kings of Rome) in 476 - 483 AD
After the fall of the Roman Empire there were other works of rehabilitation following another earthquake which occurred around 484 or 508. At a certain point the great Flavian Amphitheater was abandoned and in the sixth century used as a burial area; later, used as a dwelling.
In the early Middle Ages the Colosseum became a fortress of the Frangipane and Annibaldi until 1312, when the emperor Henry VII intervened and handed it back to the Senate and then to the Roman people.
The earthquakes of 1231 and 1349 brought other damage to the Colosseum which by now in ruins was abandoned again. The sad fate made it become a marble quarry used to build new buildings including the most famous, the Palazzo Venezia and the Chancellery. The travertine blocks were removed or those fallen due to natural disasters were used to build the Barberini palace in 1703 and for the port of Ripetta. A famous saying that was part of a description of the "pillage" of the Colosseum says: "Quod non fecerunt Barbari, fecerunt Barberini" (What the barbarians did not do, the Barberini did).
During the jubilee of 1675 it assumed the character of a sacred place in memory of the many Christian martyrs condemned to torture here and in 1744 Pope Benedict XIV consecrated it to the passion of Christ and had the 15 stages of the Via Crucis built there, in memory of Christian martyrs sacrificed in the arena.
Following the structural instability, the first restorations took place: spurs to support the remaining standing ends of the façade were built in 1807 by Raffaele Stern and in 1827 by Luigi Maria Valadier, who recomposed part of the collapsed structures in the new work. Other restorations in the interior took place between 1831 and 1846. At the same time the monument began to be freed from underground with the excavations directed by Carlo Fea in 1811 and 1812 and with those of Pietro Rosa (1874-1875).
In 1938 and 1939 the underground structures of the arena were completely excavated, partly altered by reconstructions.