Complex sentence: For a territory as large as the Roman Empire, there were amazingly few soldiers, almost all of the legions were based close to the frontiers, any hostile band that managed to pass this outer area of defense could travel within the Empire without much significant resistance.
Split into simpler sentences: For a territory as large as the Roman Empire, there were amazingly few soldiers, almost all of the legions were based close to the frontiers. Any hostile group who managed to pass this area of defense could travel within the Empire without much significant resistance.