On defense:
-Use your garisson units first. Trow them into battle, whatever, they will reappear next battle ... all for free.

Beyond that I always shove my archers into the walls where the AI hits, though give priority to the side from where the AI archers come (overwelming them). As soon as the AI hits the walls in order to climb them I pull back my archers a bit and send in the melee troops to defend the walls. This way you start banging on the AI with a full unit while the AI's unit is still mostly trying to scale your walls (and again you basicly overwelm them, possibly with less troops as the attacker).
Do note that when facing samurai stacks when defending with weaker troops in a developed castle .. move your troops to the top level or else they'll end up mass-routing and at the same time draw in the enemy samurai who are more than willing to slaughter you.


On the offense, besides all the exploits, which I'm not going to mention. It seems sieges work pretty well in a realistic way, meaning that when you siege you in effect trow your entire army at the walls of the enemy, from different angles, and overwelm them unit by unit (the AI has some units here, some there and some elsewhere, get every group locked in battle and make sure you overwelm the AI in at least 1 location .. capture a gate, get your general inside the walls for moral boosts and keep pushing).
These will be hardfought battles, but I personally find them a lot of fun. I rather have the losses than wait for 20 minutes for me to get enough archers in place to shoot down the AI from the rear (you absolutelly do not have to exploit the poor AI programming in order to win).