Salve (greetings) to all. I hope this will help you in your tyrannical conquest of the world, or your noble struggle to stave off foreign invasions of your lands (A common reality in mods such as RS and IBFD).
First off, why I'm posting this; I have noticed that many people are either new to the Total war series, or coming back to RTW from MII (as I did), and while there are many posts such as "How to deal with x,y or z" there are not as many easily accessible tactical guides to using an entire army to your fullest benefit.
This will not, in the first post, be a "How to" for certain situations, but rather a primer to get you to think when encountering an enemy.
Whether you are like myself, who likes to use my favorite generals in a role-play mode, and take into account the lives of your men, or you simply want the army you've paid for to be available the next turn to fight again, there are many reasons to learn good strategies as a whole, from the movements of the campaign map to the battle map conflicts.
On the campaign map, the terrain will be reflected on the battle map. Historically every great general not only used the terrain to his advantage, but also, when he could, chose that ground. To put your army to it's best use, you should too. Total War is a great game that allows rewards for good placement and makes you pay for poor placement.
Now remember this; when you attack/are attacked, you have a choice; auto resolve or manual battle. Unless an army of vastly inferior troops in poor numbers attacks, you will get better results in manual; and you get the chance to command large bodies of troops on the ground.
Now, while in deployment stage, the computer will have automatically placed your army. I never use this default, but sometimes I don't change it before going into battle. The reason? I will be maneuvering when the battle starts, and so it doesn't matter how or where I start out. However, group your men as you feel the situation dictates. Why? The series of 8 formations opened to you when your men are grouped can be very helpful for keeping your men in good order in the battle.
Certainly, the enemy you are fighting will dictate how you maneuver. Hellenistic armies are slower, have a more powerful front, and generally few cavalry. Barbarians will have more beserk-able units, and are generally keen on charging you right in the center. And Roman units will attack in well maintained narrow yet deep formations, with a solid reserve that makes flanking them difficult.
Also, you may say, the army YOU command dictates certain tactics. Hellenistic armies under human control have a variety of tactics availible to them. Roman armies pre-marian are flexible to an extent Hellenistic armies are not, the reserve units of triarii and Principes being available to either plug holes in the front, guard the rear, or turn a flank.
My concern is not so much battle tactics after the armies engage, but before engaging.
Take the following into consideration on the campaign map: The enemy force makeup. Intelligence is important, and always has been, in war AND peace; why should a TW game be any different? Once you are about to engage, you can right click on the enemy commander to see his force. That's good. Use it to draw up a plan in your head.
Then, when on the battle map, and the battle is started, look at the enemy's deployment. You should ideally look around the battle map in deployment phase and overlook it; are there hills? Thick or thin forests? Sand dunes? All these features can be used to your advantage-or against you, even by an AI player.
When I deploy, I place my army on a defensible locale. Just because you are the attacker doesn't mean the enemy will be keen to just let you take your time and attack at your own pace. They may try to blitz you, and if your in a poor stance to recieve the enemy, you won't have time to re-deploy well enough.
If the enemy general is 4 or more stars, I've found even AI generals on teh defensive will occupy hills or forests; hills tiring your troops out faster, and forests being detrimental to both legions and phalanx units. They also nullify the advantages of missles.
Now the battle has started. You are the attacker, and you have heavy infantry, cavalry, archers, skirmishers, and some light artillery. The artillery is slow; and the enemy have deployed outside it's range. So you move it up right? what does the rest of your army do?
I can't tell you the best WAY to maneuver, since no two battles are the same. I CAN tell you that you NEED to maneuver sensibly. Charging in with heavy infantry nullifies their strengths and brings out their weaknesses-if you do it wrong. I've seen my friends and brothers charge poor enemy lines with urban cohorts in vanilla, and even though they won, that cohort was decimated. Sometimes, it cost them the battle when it shouldn't have.
Think! Move a "group" of infantry to the left, to the right, wherever they won't be opposed. Move your cavalry around the enemy. Make them move to counter the threat, and weaken their front line. Make single units detach from the line to chase you, and slaughter them. Weaken them as much as possible with arrows and missles (keeping care to watch the clock) before closing for the kill.
I may, if there are certain questions, update with certain maneuvers pertaining to certain situations. Feel free to add your input as well.
Thanks for reading.