Can someone please explain the difficulty in detail?
So far I've played on hard, went far but not too far! Full AI stacks everywhere!
Then I tried normal mode which was so easy, conquered half of Japan with the Shimzau clan.
I googled and found this:
Easy AI Difficulty Level Explained
The Campaign AI is slightly limited and will not target you as a primary in any great detail.In Shogun 2: Total War, a good place to start if you have no clue about TW games as the player gains a reasonable bonus to most aspects. The Battle AI is fairly restricted with the player receiving a morale boost.
Normal AI Difficulty Level Explained
With this setting, the Campaign AI is on full mode but still does not act overly aggressive towards you. The player gets a slight bonus towards most elements of the campaign, such as revolt levels, tax revenue etc. The BAI is still slightly restricted but no-one gets any bonuses.
Hard AI Difficulty Level Explained
The BAI and CAI are both on full and the CAI will be quite aggressive towards you. The AI gains a very slight bonus to most things in very minor percentages (such as morale, troop training costs and so on).
V. Hard AI Difficulty Level Explained
This is where the AI is on total riot, it is highly aggressive (although not stupidly so, it still plays a fair game, but affords you no respite). The AI gets minor bonuses in both Campaign and Battle. This is the AI difficulty level that will really test you and allow you to experience the full fury of the AI.
Legendary AI Difficulty Level Explained
The legendary mode is very hard but with additional limitations, such as no save game, no minimap, camera restrictions etc. The AI doesn't receive any further bonuses other than what it gets on V. Hard.