Always scout the enemy prior to moving your main armies.
Do not rush in battle at the first enemy encounter. Maneuver! Seek for tactical and numerical advantage.
Use combined arms. Single unit type is weaker than smart combination of units.
Prefer attacking wherever possible.
Prefer moving in large groups of units. Even if it seems safe - it's not.
Group armies together. Single army even if it is very strong is not sufficient to attack. Even if it wins, it may suffer heavy losses and will be an easy pray for enemy reinforcements. Having several armies in one place greatly increases your power and puts you in tactical advantage. Even if your armies are weaker than enemy's you can concentrate your strike on one enemy army and completely exterminate it, which is what you want to do most of the time. The law is simple: regroup after attack to try to save your units and do not let enemy to regroup after fight. Initiative is a key to victory.
Aim at killing, not damaging. It is better to kill one unit than wound three units. Sometimes you will have no choice but to attack an army that is stronger. It may be wiser not to spread your good units, but keep them tightly packed. In that case they will breach enemy formation and even victorious your enemy will suffer casualties.
Care for each and every unit. Give healing to wounded and food for those in starvation. Treasure every single soul, for even the smallest one can become a mighty warrior.
Always guard your weak units. Remember that spell casters and skirmishers need melee support. Supply wagons and catapults are easily destroyed by enemy covert troops.
Practice patience. You do not know how long the game is going to last. Try to plan for few turns ahead.
Broken formation is no good at all. In battle guard your flanks well. It is usually a good idea not to put weak or wounded units on flanks (especially when setting up castle defense!). Center is also very important but to a slightly lesser extent than flanks. Firstly, skirmishers will start to concentrate fire on edge units. Secondly, even strong units do not stand long if they are engaged in melee with two or three enemy units.
Use "Titan Fights Titan" rule. Do not attack weaker enemy armies with your main force because you will lose it on counter attack. You super army always should target enemy super army. Always. Even if you lose you can finish them with you disposable troops. Even if you army gets destroyed next turn, your mission was accomplished - enemy super army is destroyed also.
Castles are tough. When properly defended and upgraded castles can withstand up to three heavy assaults. Think twice before attacking.
Castle siege is a pain. Try to isolate castle defenders by closing the gates with your armies. Kill possible reinforcements. Destroy villages. Bomb with catapults. Bring a supply wagon - siege can be very lasting process.