Which is one of the revolutionary developments in roman architecture?

One of the most revolutionary developments in Classical Roman architecture is THE ARCH, and how the Romans used it to hold up structures without the use of the previously popular post-and-lintel system. The Romans used arches to create BARREL VAULTS which were tunnel-like arches, when two of these barrel vaults were intersected at 90 degree angles a GROIN VAULT was created which could be held up by only four piers if needed, which allowed for maximum stability with the least material needed. 

Ithink that'd be 4, sorry if i'm wrong.

Is there a question?