Why is the Colosseum broken?

The Colosseum is broken for a number of reasons. The short answer - main reason is an earth quake during the 1300s knocked down the main wall.  The long answer it took a beating from different factors over the years and was destabilized. 

Maintenance on the building ceased during the 5th century.  Numerous invading tribes dismantled the Colosseum - removing clamps, facades, and more that kept it strong. In the 19th century bricks were used to stabilize the structure.  For this reason it still stands today. The colosseum is a testament to the strength of old Rome. This building has stood for 2,000 years through vandals and earthquakes and most of it remains.  The mortar actually gets stronger each year!