Yes, the 3 scenario's deffinitely differ in diffuculty. Yoshitsune is the hardest, Yoritomo is medium, & Kiyomori(I think that was his name) is easy. I'm not big into these type of games, but I got absorbed in this one, so, my skills on these type of games aren't noteworthy.
IIRC, one standard strategy I would use in the battles, is to have the archer's on auto fire, & then I'd have 1 or 2 groups of soldiers move out into the field to lure the enemy towards me, & then head back to my spot with the rest of the army, with the enemy getting blasted by my archer's, & destroyed by my soldier's.
Heh, one of my friends bitd, was so into this game, that he played it, even though I didn't have enough room in memory to save my game. So he played it all nite. I remember falling asleep around 11 or 12, & woke back up around 6 & he was still playing!! Ah memories!
Edit-Ah, BT got to it before me. I think on the castle's you really just need to get used to the layout, though, it might be that castle layout's are effected by the number of enemy soldiers? I don't recall for sure. The ninja's, like BT said, it rarely happens, but you do get the hang ot it, & yes, save often! And timing is important on the the invasion's, I basically would time it so that an enemy would pop his head over a wall, just as the arrow hits that spot. It's funny, the horseback stuff I used to have no problem with, but I recall bitd having a hard time killing that leader. I have no real suggestions, other then avoiding rocks & tree's n' stuff so your horse doesn't go apespit on ya.