Roman architecture was strikingly innovative for the time and was an enormous factor in the success of the Empire. Here's a bullet-pointed list:
--Aqueducts. Aqueducts allowed water irrigation to flow into the city of Rome and throughout the Empire, allowing it to thrive. It's the same system we now use, as it builds up civilizations in previously more remote areas, as well as brings people into more urban environments. Further, it was a marker of the Roman Empire and as such established everywhere the aqueducts touched as parts of the Roman Empire (you can still see ruins of them today).
--A socially-centered city layout. For all citizens of the city of Rome, whether rich or poor, amenities such as public bathhouses and gladiator rings were within walking distance and accessible to all. Forums, which were crucial to every Roman city, established a central area for diverse social interaction and political discussion (the name itself is indicative of the place's impact). This system of an emphasis on public infrastructure promoted not only a furthering of thought and pride in the Empire that were characteristic of Rome, but also established a legacy of social-focused design that would lead (Middle Ages excepting) to what could be considered the foundation of modern democratic thought.
--Hypocausts and aqueducts/bath houses that improved water access led to an improvement in personal hygiene that raised the standard of living for the Roman people. This is a factor in an of itself, but also led to more time being able to be spent on philosophizing, politics, sport and socializing, which led to a development of human thought and a more holistic construction of the Empire.
--Roads. Arguably the distinguishing architectural development of the Roman Empire, a network of roads throughout the Empire connected its peoples, allowed for easier transportation, and heightened its influence (this is from where we get the idiom "All roads lead back to Rome."
--Monuments. Ancient Rome represents the first time artistic and monumental structures were developed in Western civilization.
