I will use an example of the Romans as this is what I am most accustomed too, although I know most other societies had a very similar method. Basically with an army the size of 30-40 thousand as you are talking about a general would devise a strategy that would be known throughout the ranks below him such as the Legates and Tribunes. These other high ranking positions would as well contribute to the plan and strategy an army will take on the battlefield but in essence the general had the highest command. To explain this most clearly I will briefly explain these high ranking positions as it is in relation to your question. I have already covered the role of the general but he was in charge of the Legates which were a rank down from general but were essentially a general in their own right as they commanded a Legion (5400 men) so the army strength your question relates to would have around 6 legions each headed by a Legate. Thirdly there were the tribunes which essentially helped the Legate lead a legion. Within these legions were centuries (80 men) which were commanded by centurions who would actually be very present on the battlefield. Assisting the centurion would than be the signifier which would have various tools such as whistles, horns and flags which would tell the soldiers what to do and when. So as you can see from just the hierarchy of roles in a Roman army it did all come back to the general who would inform his legates of the strategy and than the tribunes which would then tell the centurions and than on the battlefield the signifier would be telling the soldiers what to do when the centurion wanted it. In addition I'm sure the men would have been told before hand what the plan was and they knew from training which formations that were going to be used so the signifier was more of use to tell them when to do what. So to answer the question directly, Yes, some large armies were in fact extremely organised if all went to plan and soldiers were disciplined yet in today's standards probably not as news to the front line was carried by mouth or sound and things could often get confusing in the heat of a battle. An interesting topic to study however would be barbarians in battle as you probably know they were great fighters yet lacked the discipline and strategy other societies had such as Rome and Athens.