Pardon the title, it seemed appropriate.
The question: Would Roman-style legions and/or Hellenic phalanxes.
Elaboration: Take a medieval country, take it's serfs and feudal lords and equip them with either Roman Legionaire gear or phalanx-style gear. Use matching tactics. How would they do?
I'm not the ultimate authority on the subject of history, but after reading some books on medieval times and Roman-Greek history, I think they might do well. Just might.