Monks:
Try to mix a monk or two into your army at all times. It may not seem much of a difference, but trust me, it does make a difference. After a battle, or if the enemy retreats, try to make sure you have some monks to heal your whole army so when he comes back, you will have a healthy army to fight him with. Also, if you are the one that is retreating from a war because you know you cant win, then make sure you retreat into a place where your monks are.

Relics:
ALWAYS try to collect these. Whenever you have a idle monk around (which you should at some time because you should be making those monks!), use the monk to collect relics near you. They may not seem much, but over time, they will prove useful. Another tip; try to build another monastery somewhere safe for example, behind your base or within it. You don't want to collect 3 relics and put them in your forward monastery because it could get destroyed VERY easily and then you would of gave the enemy those 3 relics. So having 2 monasteries, 1 in safe place, and 1 in forward base is a good idea.

Petards:
Not a very common unit used by people. These crazy men that a willing to explode themselves are not only brave and stupid, but they are also very useful when it comes to surprises. Ever had one of those castle wars where you build a castle in front of a enemy castle to defend or vice versa? Well, the castle age solution is petards. Make about 9 petards and make sure you garrison them in the castle so your opponent don't know what your up to. That will cost you 710 food and 180 gold. Put castle gather point to the enemy's castle and then un-garrison and BANG! Gone. Taking out a enemy castle in a flash for 710 food and 180 gold in my opinion, is well worth it. The reason I think its worth it is because food is not too much of a problem and when castles are having wars, its usually a bit later in the game where you have plenty of food lying around so that's certainly not a problem. The 180 gold is a great deal. That is very cheap, its like one of those bargains in the Sunday market. Another useful way to use them is to make 5 of them and charge them into the enemy's town centre mixed with a couple of knights. But make sure your knights get there first to trigger the enemy's reaction and make him garrison his villagers, then in come the petards and BANG! Again, then there will be 15 villagers all bunched up for your knights to kill. The enemy wont expect that which is why it is such a great technique. And lastly and most obviously, when there's layers of stone walls, petards do the job much better than rams or mangonels. Petards also take down trebs, you need about 2 petards to take down a treb, but you will need some units to accompany them as the enemy's trebs are most likely guarded.

Repairers:
REPAIR! You must repair. Damaged rams don't last long if you constantly use them without repairing. Same with mangonels, especially when taking down towers, bring a repairer to repair the mangonel while taking towers down. When you are fighting trebs with trebs, the deciding factor is REPAIRING. If you repair correctly at the right times, your treb will always be full health. Even when the enemy charges a bunch of cavalry to take your treb, 3-4 repairers might keep it alive for a extra shot or two which may be crucial as it might just be enough to take down the enemy castle. Damaged town centres also need to be repaired, don't just stand there and watch it go in flames, REPAIR IT! Have you seen a group of champions take down a town centre when its fully garrisoned? Well do you how to stop that? You repair. Un-garrison 2 villagers to repair your town centre, most the times the enemy wont notice because he will be busy somewhere else. And finally, castles. When the enemy starts trebbing your castles, don't only think about how to get to that treb which is guarded by a million halberdiers, grab about 5 villagers and keep that castle up!

Trebuchets:
NEVER leave a treb unguarded. Keep at least 10 units guarding those trebs. Trebs are costly, try to keep them in full condition (repair) and keep units on 'guard' command to trebs.

Raiding:
Try to keep raiding the opponent. Make many stables and set gather points all over the enemy's town and queue lots of hussars. I'm talking during imperial age where food is 'free'. If you are a meso civilisation, then don't make barracks and set gather points all over because eagles are expensive and you don't want to put them to waste like that. Instead, make a group of about 10 eagles and charge them to the enemy's town - this does involve more micro managing than with civilisations that get the hussar, but its worth it. Try to keep the enemy under pressure at all times.

Lumber Camps:
During the late game period, you don't want your woodcutters walking a mile to cut some wood and walk another mile to put it back. Make lumbercamps each time your villagers need to walk miles to carry it there and back. It saves time and is much more efficient.

Ranged Units:
Don't make ranged units without melee back up. Its best to have some sort of melee unit to make the enemy's units fight them while your main ranged unit wipes them out. Arbalests are great backup for champions because champions are good units that can stand in battle long so the arbalests get more time to shoot the enemy.

Claim Land:
When you take over the enemy in a war, don't leave the land there as a battle field, try to make military buildings or town centres to claim that land you have won. Its always best to build your military closer to the enemy so that your units don't have to travel so far. ALWAYS build something on the land you have won, otherwise the enemy will just take it back easily. It will also delay the enemy from advancing so quickly because he will have to get rid of your forward buildings in order to advance to your town.

Upgrades:
Most importantly, the blacksmith upgrades. You must remember to upgrade your units, upgrades could decide the battle. Other upgrades such as bloodlines, thumb ring and parthian tactics are very useful too, make sure you research them if you are going to use the units that require those upgrades.

Communication:
You must communicate with your allies in team games. They need to know whether you need help or not. When asking for help, a flare + typing "HELP!!!!!!" is not very helpful. A flare + typing "Help, he has about 5 knights" or "Help, can you send about 8 pikes?" - that kind of call is much more helpful. During the early stages of the game, communication is extremely important because your allies would not have researched cartography yet so you would need to communicate a lot.

Other things to keep in mind:
* Hotkeys
* Try to keep villagers at work, an idle villager is a very bad thing. Use the '.' hotkey to keep those villagers at work.
* Town Bell - DON'T use this unless your whole town is under attack, try to highlight all the villagers nearby the town centre and garrison them instead. Learn to garrison instead of ringing the bell.
* Towers - When you make a tower, don't just leave it like that, make palisades or even stone walls around them to make sure the enemy don't use a bunch of villagers to take them down when they are unguarded. Make houses or other buildings around them too.
* Hold shift to make multiple buildings, very useful when making farms.