It all comes down to a mix of big strategic thinking and smaller tactical thinking. They must be combined into a powerful tool to use.
Strategy: attack the weakest; attack those nearby; outnumber the enemy; attack when it suits you; have backup plans or prevent disturbance of your plans.
Example: if you besiege a settlement have an extra army near to repel any relieving forces, so you can starve out the city/caste. When they sally out, make sure you have good missiles to defend, making any approach useless.
If you plan to conquer somebody, prepare a superior fleet blocking their sea trade to damage their income. Do them all at once and they will suffer.
If possible, move armies in coordination with each other: one offensive army and one defensive. When conquering all over mainland have more armies ready and move in coordination with each other to reinforce and flank if necessary.
Divide and rule. Destroying little pieces and chunks of the enemy will wear them down in the end.
Sometime to attack is to defend. Sometimes to defend is to attack.
Regarding tactics it'll be more about battles themselves of course, but I can't say much about it, 'cause the player would have to react to the enemy's action and vice versa - it can be different any time.
My personal favorite army setup:
-General
-Heavy Infantry (mainline)
-Spears (guard flanks against cavalry)
-Heavy Cavalry (each side to flank/rear mainline, or to repel enemy cavalry, then move to mainline and rear)
-Long range missiles: archers and artillery to inflict as much damage as possible
Defensively, with this setup you can do a lot in my opinion, if the enemy approach. And offensively, you can do a lot too. You can fight a mainline battle and flank, repel cavalry, and shoot down annoying missile cavalry with your archers.
Strategically, always pick the best position (hilltop, end of a bridge defending, etc.) to give you advantage. I once had my Pavise X-bowmen atop a HIGH hill as my crusading army marched forth to engage the enemy. Their missiles continuously flanked the enemy. Also, it'll be useless for them to try to approach these missiles, as they're too far, and their stamina will be drained.
Tactically, hiding units at trees is a good move too. I once surprised the enemy turning the tides with a quick heavy cavalry charge into their flanks.
That's all I can think of for now.
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