How did the Romans effectively combine their whole empire into one unit?

There are two different aspects one needs to consider when talking discussing whether a civilization was effectively combined into a single "unit". First there is the physical aspect. Rome was a very large place, making getting from one place to another rather difficult. Rome greatly assuaged this problem by installing a number of roads. Not only did this make getting around a lot easier, both for the common person and for the army, but it also steeply increased trade, causing Rome to become even more wealthy. The second aspect is the metaphorical one. For some civilizations, such as the USA before the civil war, all of its citizens were, let's just say, not all on the same page. But Rome's constant warfare gave all of the Romans a common goal, and a common enemy. This brought the society together, and the Romans put aside their differences to focus on this one goal, and this one enemy.