The Colloseum is still standing today. It was in use at one point as an arena, and then it wasn't at some point. When was its last use as a a typical Roman arena? And what did medieval Italians think of it? Or during the renaissance for example? Was there some kind of active preservation through the years? Are their paintings of it that show it's erosion?
The emperor Honorius abolished man vs man gladitorial games in AD 404. There were some animal games (venationes) in 503, and in 508 the place suffered earthquake damage. Rome's population had plummeted by about two thirds over the previous century and although there were idle rich about, these spectacles were not cheap to put on. The last recorded was in celebration of the consulship of Anicius Maximus in 523. The Ostrogothic ruler of Italy Theodoric the Great (illegitimate some of Alaric the Goth) gave his permission, but also made it clear that he didn't think much of the games as an occupation for respectable men. If there were others after that, we have no record.
Which left the structure, an impressive pile of second hand building material. First to go were the bronze clamps used to secure blocks, much as abandoned buildings today get stripped of copper piping. the probable culprit is Totila, who sacked the city in 545. (Obviously the building didn't fall because of the metal being taken out, but the architects who built the thing had a large budget, so why not err on the side of safety?) Rome's population plummeted, those who remained preferred to gather by the river (no more aqueducts, and toting water is tiresome). The Colosseum went into decline, unnerving to the locals who thought it a bit creepy.
By the ninth century things in Rome were looking up again. We have evidence of people turning some of the interior space into living quarters and animal stables, and the center into a market. It must have made a nice little neighborhood, if you didn't mind the trek to the river to get water, or if you could afford the water carriers who would do it for you for a price.
Alas, 1084, Robert Guiscard comes to sack the city (see here for more), leaving in his wake a city broken down into zones controlled by various powerful families and their bully boys (think gang turf rivalry). The Colosseum fell under control of the Frangipane family. With one brief interruption (rebelling citizens, tired of the baronial families, threw the lot of them out for the period of 1144-1159 and tried to restore an ancient Republic) that family held it until the Anabali family wrested the building from them at the end of the 12th century. They in turn had to give it over to the church in 1312.
Another serious earthquake in 1349 more or less put it out of commission for shelter. Instead it became a quarry for building material, a great source of profit for the Benedictines who held the lease hold of the place. The area continued in that role for the next hundred years; much of St Peters started life at the Colosseum.
You can see woodcuts of the building dating from the fifteenth and sixteen century. Much of what you see today is restoration. Serious restoration.
(I used to live about a mile from it, and have always taken an interest in the place.)
