What does the author consider a great accomplishment of Rome?

This depends on who you consider to be the author. If the author in your case would be a famous old historian, then they would probably marvel at the architectural accomplishments of ancient Rome and how they were able to create such magnificent structures in such a short amount of time.

The Appian Way was the first military highway, this was Rome's greatest accomplishments. Other accomplishments were arches, concrete, bridges, arched roofs, and aqueducts.