Since the fall of the Roman empire, what purpose has the colosseum served that kept it standing all these centuries?
To think of a major arena or even an icon like the empire state building standing for centuries without being mowed down and replaced with something new seems unlikely... what made the colosseum so special? Was it really such a proud icon 5 centuries after the fall of the roman empire that it was held intact?
It fell out of use as an arena in the 6th century once Rome (the city, not empire) became too impoverished to support that sort of extravagance. During the Middle Ages it was used as housing, shops, and once as a fortress. Most of the other facade is missing because of earthquakes and locals adapting it for building material. Back then it might have looked a lot like the Theater of Marcellus does today.
Part of what helped save it was the idea that Christian martyrs died there, so there was a religious significance to the building. Eventually the structure came under papal protection and starting with Pope Benedict XIV in the mid 18th century there were concerted efforts to preserve the building from decay, clear out residents, and buttress what was left.
The Colosseum is, well, colossal (though it got its name from a colossus formerly standing nearby, I'm just making a pun). If you've ever walked through it, you'd feel like you were in a modern stadium. It could seat perhaps over 50,000 spectators. It'd probably take a concerted effort to actually level a building of that size, and while it was picked clean by medieval Romans the actual structure of the building was quite useful so there was no reason to fully demolish it.
