That's because the Fourth have learned the 5 D's of Defense, Dodge, Duck, Dip, Dive and . . . . Dodge. Well maybe just the first D, but still they are better than those ruddy romans.
No they don't. Colonial minutemen were the most defenseless, inefficient ranged soldiers ever. First they had to contact the enemy in advance to schedule a time and place for each battle. Then they fought in flat, open fields with no civilians around. They stood completely still in a wide row, wearing bright colors, with drums and flutes to make sure everyone noticed them. They attacked in volleys, refusing to aim with their sights because of honor or something, and the first snipers were treated as criminals. Anyone who tells me "War. War never changes." is getting punched in the nuts.
Why do the romans have less defense? Because it's a turn-based board game about magic and time travel. Leave your realism at the door.