In this part I will bring up the basic of tactics and how to create a stable economic foundation for your empire. The first thing you should do when you start a new campaign is not to go on a rampage, slaughtering and killing all in your path burning cities to the ground and stomping on the dust. Sure you could do that but it would make it far harder to keep your empire as you would gain very many enemies and trouble with your own ''loyal and content'' citizen. What you should start with is building up your economy by constructing trading posts and ports in your cities then upgrading the roads while you walk around with a couple of diplomats and secure trade agreements with as many factions as possible. Why the ports and roads you wonder? -I'm not making a fleet. Well it's because a larger port allows you to have more trading fleets under your command. (You can't control them or build them, they are AI managed.) This will improve your income and the sooner you make them the more you earn from them as they pull in more money for your empire by every turn of long lasting trade contracts. So why the roads? Because they also help to boost your trade income as well as the ports and together with the trading posts you will earn a lot more.

After making the roads, ports and trading posts you might want to start conquering a little and discard your faction leader. I'm not saying kill him! Just let him stay back home in your capital and pick out the most talented YOUNG man in your faction. Yes that's why you should leave your OLD faction leader. Make the talented youth your faction heir and don't care about the bloody lines of succession, this ensured you a good, competent leader for your faction.
Don't make a too large army when you go on your ''holy crusade'' because that will be a large strain on your economy since it still isn't that incredible. Make the troops you know you need. I'll come back to this under the title ''Strategy and Tactics''

My way of battle is a very defensive one, not to make huge armies of archers and stay behind stone walls, that's not it. I like to make troops that have a high defense and care less about their attack as I want them to be able to hold the line for a long time. I know many people say ''Don't ever use your generals in combat'' but that's just retarded! Use him to the max, not doing anything ballsy and suicidal just make sure he knows how to kill. Think off it in this very crude way, spears butcher cavalry, cavalry butcher skirmishers and archers (and are good for hunting down fleeing enemies) swordsmen are good against spear-men and if I've missed something figure it out yourself.
The thing that makes cavalry an excellent force on the battlefield is the fact that they are fast, their charge is devastating and they crush the enemies morale. Just imagine seeing 500 pounds of horseflesh running straight at you and a man on top of it hacking and slashing at everyone. Terrifying right? Well cavalry is BAD at fighting in extended battles, they are best used by charging at the enemies backs of flanks then withdrawn about ten seconds after the initial charge when the enemy starts to recover from the shock.
This is why I love to fight with a strong backbone of infantry to ''hold the line'' while the cavalry smash in to the backs of the enemy and make them panic. One thing to always remember!
NEVER completely surrender your enemy because that will force them to fight to the death causing you unnecessary casualties and delaying your forces movements to where the line is buckling for example. Use your cavalry to hunt them down when they flee instead.
Also, remember skirmishers and archers are practically useless if you don't count the few good units like Gallic archers or Britannic head throwers.

When you attack an enemy town, besiege it until it comes to you, this will make you lose fewer troops and you will also have the advantage that you can draw up your lines while they march at you in a mess, use your archers to harass them if you have any but don't let them go too far from your main force or they can be hunted down by the enemy cavalry before you can help them. Mounted archers or spear throwers or what ever as long as they are fast and ranged are the best as they can't be hunted down and can harass all they want. Don't run with your troops if you don't really NEED to as that will tire them much faster making them less efficient in combat and slower when they move. If your troops are tired after a clash of the lines when the enemy pulled back, let them stay put and rest to regain their strength before the enemy comes back. Again, harass them with ranged units. Don't forget to use your troops in guard mode so that they hold the line and don't hunt down fleeing enemies. NEVER hunt down fleeing enemies with infantry as they are slow and often very weak when exhausted from hunting an enemy and then the hostile cavalry can smash in to them and rout them.
There are exceptions to all of the rules I've drawn up but these are just general guidelines.

When defending a town you should always wait for them to attack unless it's the last turn and your garrison will all surrender if there is no battle. Let the enemy come to you! If you have phalanx units you should not post them on the walls but rather to defend the slim alleys leading to the town square/plaza/flag where they can't be flanked and thereby can deal with almost any enemy as they have to fight trough the spears and can't get around them. Then you post your ranged troops behind the phalanx and just fire at will so you have a mini-army ready for the enemy.

When you are fighting a battle that is not in a city you should always look for objects you can use while lining up your troops, for example a farmhouse that can cover one of your flanks or a hill from which you have the advantage. Bridges are probably the greatest strategic advantage/disadvantage if you are defending and respectively attacking. They are the best choke points there are and can ''easily'' be held by a smaller troop as the enemy will advance across the bridge in great disorder at the same time as you should use any ranged troops you have, archers, spear throwers or even better, artillery to fire at the massed troops crossing the bridge. If you are defending, don't draw your troops up in a line right in front of the bridge blocking it completely, draw them up like a U if you understand my point. Force them to fight in three directions with arrows all over them as they have just crossed and you have a far larger moral and actual combat advantage.

As Clausewitz said: ''A strong terrain should never tempt you in to a passive defense''
His way of using terrain was that the stronger terrain, defiles, hills, etc. should be used so that you could hold that part of the line with fewer troops then if it had been a plain so the men not used there could be used to crush the enemy elsewhere and thereby drive the victory home.

I have drawn all my lines from personal experience and common sence but here comes some guides form historical generals, actually just Von Clausewitz and Sun Tzu but I hope they help.

The first advice I want to share is Sun Tzu who says ''Put strong against weak''
This means you use your weak/weaker troops to hold the line against the enemy elite ones while your elites smash the other enemy flank and perform a pincermanover on the enemy center from the flank.

Clausewitz was a huge lover of reserves and this tactic:
Keep plenty of reserves, this will help you more then you can imagine!
Naploeon did exactally this against the fourth coalition where his left was smashed by the prussians but he sent in his reserves to hold the flank from being routed while his right flank completely smashed the prussian and drove the victory home because Blücher didn't keep sufficialn resevers to counter his move.

From The Principles of War by Clausewitz:
At Ratisbon, Marshal Davout defended himself passively, while Napoleon attacked the fifth and sixth
army-corps with his right wing and beat them completely.
Though the Austrians were the real defenders at Wagram, they did attack the emperor on the second day with the greater part of their forces. Therefore Napoleon can also be considered a defender. With his right wing he attacked, outflanked and defeated the Austrian left wing. At the same time he paid little attention to his weak left wing (consisting of a single division), which was resting on the Danube. Yet through strong reserves (i.e.,formation in depth), he prevented the victory of the Austrian right wing from having any influence on his own victory gained on the Rossbach. He used these reserves to retake Aderklaa.

First for you who didn't know you can select a troop (or several) of ranged troops like archers or ranged cavalry and if you hold alt you will automatically put them in to melee when they get their next order, incredibly useful for roman legionaries as they win't have to throw a spear before charging this way of using ranged cavalry to hunt down routing enemies.

Fire arrows are more worthless then you think, they have a poorer accuracy and slower rate of fire then normal arrows but they are good against elephants and chariots but that's about it.

When there is an outbreak of plague, make sure to keep all of your units and agents in the city if it's yours. Isolate it as much as possible and then more your agents to hostile towns so that they get the plague.

When a town is fully upgraded and you're starting to get tons of problems with overpopulation and such gift it to a hostile faction then take it back the same turn and massacre the population, problem fixed.
You can also spam peasants there and move them to other towns and disband them there=more population there, less in the large city.

The Wedge formation is the first I'll bring up as it is one ability with far less power then you might expect. It is meant as a was of charging in and splitting the lines of an enemy force to punch trough and split them in to two parts which you can fight more efficiently. This ONLY works if you have stronger cavalry then the enemy has infantry. Otherwise your cavalry will just be stuck in the enemy line, surrounded and killed to the very last man. It is also a bad idea when you have a formation-unit or even a regular unit fighting the one you want to charge (Ex. a phalanx unit) as your troops would slow the cavalry down so much their charge lost its power and would then be trapped betweed the enemy and your infantry. It can be used by most cavalry if they aren't ranged (I think they can't be ranged) and if they have no other ability.

The ''suicide'' ability
It can only be used against elephants when they are panicing but should only be used when your elephants are potentially hurting your own troops, instead of killing them, just slit their throats with the ability and save men and time. (Or what ever the men do)

Cantabrian Circle
This can be used by ranged cavalry and only if the unit has no other ability. It's great for killing hostile missile troops as it makes your troops difficult to hit. ONLY use it when your troops are taking fire as it is only a HUGE waste of energy to have them running all the time instead of just when they need to.

The classic, the Phalanx formation!
It is probably the most wellknown and loved ability in the game as it can make a weak unit like greek militia hoplites a quite hard unit to kill if you can make sure the phalanx in not flanked or stabbed in the back as that is devestating to their power and morale. It's unnessesary to keep your units in phalanx formation if you're moving and there is no threat as it tires the unit faster and makes it impossible for them to run if it's acute.

Firearrows/missiles are less accurate as I've said and have a slower rate of fire then the normal ones, the only times you should use them are: Against chariots to make them panic, against elephants for the same reason, against siege equipment to burn it to ashes and sometimes against poor troops like militia hoplites to make their already low morale plunge and panic.

The Warcry ability is AWESOME! It can be used by nearly all barbarian units and gives aproximatly ten extra damage and a large morale boost after being used for about ten seconds! Use it before attacking if it won't put you at a disavantage to attack fifteen seconds later.

The berserkers warcry is also a great ability but don't use it too soon or the unit will split from your army and get killed without any support. It gives the unit the normal warcry bonus instantly and also makes it impossible to rout them.

The Testudo is great for defence against missiles but a unit CAN'T FIGHT IN TESTUDO. It also has the downside that it tires the unit quickly. Watch out for units firing at you from behild as that is the weakspot.

The generals ability to rally troops is a great ability for several reasons. First if you're charging your generals bodyguards at an enemy unit that will be a hard nut to crack use the rally troops right before you slam in to the enemy lines as the bodyguard will keep going but your general stop to rally his troops keeping him out of the first charge and at the back of the lines. Secondly it has quite a short range (well don't waste it at least) so use it quite close to the troop/troops you want to be effected by the morale boost and it also has a chance of rallying a fleeing unit to you making it possible to command again.