Strategic Wisdom

How To :-
Conquer an 'Impossible' Province!
Effective Castle Placement and Use!
Garrison a Castle!
Launch a Seaborne Invasion!
Reinforce a depleted unit!
Exceed the Unit size!
Track am Emissary, Priest or Diamyo!
Recon!
 

When you're facing a tough battle with defenders holding a river or a stubborn hilltop position, you're often better off throwing overwhelming numbers at him and using the "Resolve Combat Automatically" option. 
This generates the outcome through a formula that puts more weight on numerical superiority than tactical advantage. Try to get odds of 3 to 1.
This is also a good way to use hordes of Yari Ashigaru; throw them at a province and wear it down using the automatic combat resolution. 
Cheap, yes but so effective.
 

It's not necessary to keep border territories garrisoned with large armies if you use castles wisely.
Castles are less useful for any direct defensive benefit during a battle than they are for stalling an attacker until you bring reinforcements in.
This is how you can effectively park your main army one or two territories away from a potential threat. 
Keep a castle in a border province and garrison it with a token defence.
When the province is attacked, hole up in the castle until your main army arrives to break the siege and hopefully destroy the aggressors. 
 

Garrison a Castle
Select an individual unit from within an army and drop it in the castle.
You can't drop multiple units - nor can you drop a single unit army into the castle - a design flaw I think.
 

Building a Port in a province makes it effectively border every other province with a Port, no matter who owns that Port. However, you must verify the presence of the target Port before you can move an army there. 
You can do this with a recon unit, like an Emissary or Shinobi. 
Simply try to move one of these units from one of your provinces with a Port to another coastal province. 
If there's a Port in the target province, you'll be able to drop your unit there.
The next turn, he will have confirmed the Port's location and you can move armies in. 
Using Ports as avenues of invasion is especially important in the endgame, when you have to do a lot of long range manoeuvring and mopping up.
I have never seen the computer player invade this way.
It opens up a whole world of possibilities!
 

You can reinforce a depleted unit by dragging and dropping it onto another depleted unit.
The maximum unit size is determined in the Options-Performance screen.
There's a slider here labelled Default Unit Size. This is set to 60 unless you've changed it. This means that newly trained units will begin with 60 men and you won't be able to combine two units if they add up to more than 60 men.

But when you increase the setting, Shogun will allow units containing up to 120 men. This takes effect immediately and can be changed at will with no effect on stacks you've already made. 
If you change the setting to 120, stack two 60-men units into a 120-man unit, and then change the setting back to 60, you'll still have a stack of  120 men.
Maximimum army size is still 16 times unit size, ie. with the setting at 60 men an army can hold 16 units of 60 men or 8 units of 120 men.
The advantage to all this? Patch the game to v1.11 and try to get the Legendary Swordsman event with unit size of 60. It Won't Happen!
Too many casualties. 
Plus you'll lose Great Generals and Heirs with frightening regularity.
So combine your Generals with other units to keep them safe, and having your samurai running around in units of 120 will (almost) guarantee being able to build No-Dachi Samurai.
Why play with a unit size of 60? When you are fighting battles on more than one front the flexibility and rapid builds are far more important to me. 
Having the unit ready for deployment next turn and being able to send 60 men to a province may stall the assault that you know is coming. 
Rather than wait 2 turns and expend 120 men trying to retake your province.
Plus if i'm low on koku I can build 60 ashigaru easily.

Having trouble with a Diamyo who moves all over the place when you want an audience?
You can drag a unit onto a Daimyo from any territory and it will track the target and move, turn by turn, until it reaches it. 
Even though you can't see the Diamyo your emissary, ninja or geisha can.
The same holds true for assassinating any target - once you see the target set your dogs on it. From any province on the map! They'll hunt him down!
Be wary of setting more than one assasin loose at the same target at the same time. If your first assasin kills the target the others are likely to track the database character slot. (a glitch)
Meaning your ninja goes rogue and can wind up on the far side of japan attacking total strangers! The odds of him running all the way back to base are remote - unless he happens to be in a province with a port.

Emissaries and Priests are good for reconnaissance, since they can't be caught and killed by traditional defences like Border Forts and Shinobis. 

Use them to determine just how many troops your opponent has - it's amazing how good the computer players are at appearing to have fewer or less troops as the situation calls for. 
The Hojo are excellent at 'hiding' their numbers.
The computers players are also very skillfull at shuffling units. 
Especially the Oda!
Their goal is to trick you into positioning your border forces in such a way that you are vulnerable - in an even fight the defender wins. 
If they hold your province, even if you hold the castle, They are the defender!