Rome was one of the strongest empires at it’s time. At it’s peak the Roman empire stretched north to modern day Scotland, down through Europe, east into Asia, far as the border between modern day Iraq and Iran, and its southern reaches extending into northern Africa. The Roman empire also was 1,699 million square miles. The empire was invaded by Barbarian tribes, some of them were the Saxons, Vandals, Visigoths, Ostrogoths, and Alamanni. Rome went into war and got Barbarian mercenaries, which are foreign soldiers who fight in the military for pay. Then there's the Byzantine empire one of the richest it's time. They had great defenses like the Theodosian wall and seawalls. The Theodosian wall stretched from shore to shore, with room in the
