The Roman civilization was one of the largest empires of its time, spanning over 3.5 million square miles across North Western Europe and the Mediterranean Sea. At its height, the empire had an estimated population of 9 million inhabitants. The Romans differed from the Greeks in that they controlled a larger unified land area, allowing for stronger central government. The Romans were also more intent on imposing their culture fully on conquered lands, rather than establishing colonies like the Greeks. Key factors in Roman architecture included abundant building materials, the development of concrete which enabled larger structures, and adapting to different climates across the empire. Religious practices changed over time from the Etruscan period to the imperial period when Christianity grew prominent. Roman civilization had a