It's incredible to think that this imposing building took less than ten years to build. How did they do it?
The Romans were really good at an architectural technique that they knew well: the arch.
An arch allows you to distribute the weight of heavy constructions effectively, in a perfect manner. The Romans used arches as the main element of their architecture, think of Roman aqueducts, for example.
The Coliseum can be viewed as though it were a series of aqueducts built one on top of the other.