The structure itself carried a particular characteristic of Ancient Roman architecture: arches. The Colosseum's first three stories were basicly arches and columns. The decoration on these walls were more elaborate than the ones on the fourth story which was added later on. The first story was mainly entrances, but the second and third stories were decorated with a statue in each arch. An interesting fact about the building is that the fourth-story walls held poles that conatined awnings to shade from the sun and guard from the rain. The entire structure was formed by brick and concrete, a typical ingredient in Ancient Roman architecture. The Colosseum had 78 entrances for the plebeians and 2 reserved for the emperor, giving a total of 80 entrances.
 
 

The pictures shown above gives you a very good view of what it looks like inside the colosseum.