This is one fascinating subject indeed. I am not an expert in this field, but what little knowledge I have, I will share - One must never think that an old (pre gunpowder) battle is a single, huge, bloody fight, that can be resolved quickly, no - it is more like 20, 100, 1000 small fights, 2 - 10 minutes long. The big picture, battle, can easily rage on an area of several square miles (I don't know the accurate figures, maybe much bigger areas) and take (if lucky) hours, or days to finish. Usually, a meleé (a fight, of, say 1000 men) ends when one side simply breaks off, and retreats in order to fight another round: this happens normally because the enemy formation can't be broken at the moment, and prolonged meleé would only result in casualties and fatigue. Meleé combat is seldom conclusive, and retreat maneuvers are carefully drilled, and normally the enemy can't give a chase without breaking their own formation, or giving away their strong position (high ground, a chokepoint, etc.). Ancient battles are mostly just a matter of which side can keep their formations together, and break their enemies formations, after which, they can be chased and slaughtered by cavalry or such. Once broken, a formation is simply said: broken! Useless. If an unit can't hold it's formation, it is next to worthless, no matter how experienced the soldiers are. It is ironic, indeed, that many cultures used to fight in "broken" formation. Imagine a broken mob (gauls, germans and such) of 1000 against a shield wall of 200. Assuming that the shield wall isn't flanked, and that the soldiers are well trained, well fed, well paid (<--IMPORTANT!!) and not too tired, they are quite likely to hold, and when the frustrated mob notes, that they can't break the wall, they retreat, and try take a breahter, then rush in to the fray again. If they are chased by cavalry, they are likely to panic, however. The controlled retreat turns into a panic. The panic spreads, and soon the whole enemy army is on the run. Light cavalry is generally used to chase down these routers, and capture or slaughter them. This is where most of the casualties are inflicted upon the enemy - after the formation is broken. Battles themself are normally inconclusive. Imagine how hard/slow it is to actually to kill a man with a sword, spear or a greek pike or such - from the front, that is, and as a part of a battleline. A sword, a sharp piece of metal, that is as fast as your arm is, and as strong as your technique, training, and yes, your arm is. Now imagine, that there are, what, 3 spots where your opponents armor can be pierced, and without letting your own defense down. Imagine, that you can't move, because you are flanked by friendlies from sides and behind, and the enemy in front of you is equally stuck - neither can do what they want. They can only stand still, and wait for the order to retreat, and see if the enemy is stupid enough to follow. Killing? Hard. Imagine the same scenario when your enemy has turned his back, and possibly dropped his shield and helmet in panic. Something has happened, perhaps the stench was just too much? Just make sure you don't see his eyes, or you might pity him. Killing? Easy. It is the rout that is bloody, but many examples exist of battles, where ground and maneuver were beatifully used in order to deny opponents ability to fight in a formation, and great many casualties were inflicted upon the enemy without a rout. (Gaugamela and Cannae, two battles that are always worth studying). And this talk about people being physically different 2000 years ago? True. But it's not the genes, but the fact, that many people worked in the fields and otherwise, with their bodies, rather than their minds. They were used to being cold, hungry, scared and frustrated, and yet do heavy work or risk a beating or mutilation from their superiors. Average day in the legions was mild compared to average day in the fields, or such - hard work, harder discipline, no food. Hard drills, harder discipline and reasonable food, in the legions. In the few battles (if any, many soldiers never saw a major battle, or a battle at all) they participate, they would mostly march around and hold the line, and when the enemy is tired enough, attack in force along with cavalry and missile support.
It is no surprise, really, that a soldier who is trained in modern, western mobility and "press button" warfare has trouble understanding how old warfare works.
(This is only my vision of how old battles were fought. If you see anything to correct or add, please do. I'm quite interested in the subject myself, and would love to see some conversation.)