After completion in 80 AD, lightning caused a major fire in the upper wooden levels in 217 (when it was 137 years old). It was not repaired until about 240 AD with additional repairs in 250 and 320 AD. In 443, an earthquake caused major damage that was repaired in 484 and 508 AD. Gladiator fights were last mentioned in 435 AD, and animal hunts continued until 523 when Ancious Maximus was criticized by King Theodoric the Great for their high cost.
Beginning in the 6th century, the Colosseum began to undergo changes. A small church was built into the structure though it did not apparently add religious significance to the entire Colosseum. The arena was converted into a cemetery. The vaulted spaces under the seats were converted to housing and workshops that were rented out well into the 12th century. The Frangipani family enhanced the Colosseum and used it as a castle beginning around 1200.
A large earthquake in 1349 caused the outer south side to collapse and much of the marble from that collapse was used to build palaces, churches, and hosptials.
In the 14th century, a religious order moved into the Colosseum and remained there until the 19th century. During this period, stone was stripped frm the Colosseum and the bronze clamps that held the stonework together were pried and hacked from the walls, generating the pockmarks we see today.
Pope Sixtus V (1585-1590) wanted to turn the building into a wool factory to provide employment for Rome's prostitutes. In 1671 Cardinal Altieri was going to use it for bullfights until the public voiced opposition. In 1749 Pope Bendict XIV stated that the Colosseum was a sacred site because early Christians were martyred there. (There is no evidence to support this claim, as stated in the Catholic Encyclopedia from modern times). On the plus side, he forbade additional quarrying of the Colosseum.
Later popes attempted stabilization projects, removed extensive vegetation that threatened the Colosseum stability, and reinforced the facade with triangular brick wedges in 1807 and 1827. Interior repairs were done in 1831, 1846, and the 1930s.
As always happens, my thoughts on civilizations and cultures changes when I visit a region. I arrived in Rome very angry at the Catholics for plundering the Colosseum, by robbing it of marble to construct the Vatican and other buildings and neglecting it for centuries. But as I listened to the stories and thought about how large a time span 2,000 years is, I came to realize things I had never thought of.
First, even the Romans robbed other buildings of materials in order to build their temples and theaters - they were certainly no angels either. And the Catholics didn't just destroy buildings of other cultures not their own, they destroyed their own buildings...look at how they stripped the Tower of Conti of its marble. Also consider how Via della Concilazione, leading up to St Marks Square, was built by destroying Church of S. Giacomo a Scossacavalli and Church of the Nunziatina. Of course, Via della Concilazione was built under orders from Musollini, but the Vatican helped develop and approve the plan.
It appears to me that prior cultures did not value great buildings as much as our generation does. I was also impressed at how many artifacts of prior cultures and religions are preserved in the Vatican itself. This is a good sign, in my opinion, though a skeptic could view it as plundering other regions of the world.
Finally, it dawned on me that, even had the Colosseum not been stripped of materials, it would still be in very dilapidated condition or very heavily restored over the centuries. The little original marble that remains is in deplorable condition, and that would have happened no matter what. As would have the earthquakes that severely damaged it - there was no stopping those!
Shoot, I can't even keep my own house from deteriorating in as little as 5 year's time!
So I'll go back to appreciating what is in the here and now. And wishing I was a multi-billionare so I could build and exact replica of the original Colosseum on a site somewhere removed from these fabulous ruins.