Roman architecture was strongly influenced by two great inventions - concrete and vaulted arches

Concrete was a recent invention when the Colosseum was built and the Romans were still learning how to use it. As concrete was so new they did not know how strong it was or long it would last. The Romans cautiously combined concrete together with stone. Concrete was made by mixing a strong volcanic material (called Pozzolana) with rubble, sand and a mixture of limes. The Colosseum had four tiers. The ceilings of the passages and corridors which circled the arena on each tier consisted of vaulted arches made of concrete but the supports they rested on were made of strong, heavy limestone. The Vaulted arches made the ceilings much stronger than a flat ceiling would have been. Vaulted arches made of concrete added strength to the building without adding excessive weight. Without concrete and vaulted arches, the Colosseum could not have been built.