Well if I remember correctly Ashigaru where essencially the japanese equivilent to Militia or Peasents recruited forcefully. Now I don't know if Shogun 2 has equivilents of those already, but if they don't then it makes sense that they break that easily, they did in previous games usually as well.
That's pretty much it exactly - Ashigaru are more-or-less conscripted peasants. Yari Ashigaru are equipped with spears in large part because there's very little training required to use a spear: "Stand in a line. Pointy end goes this way." They're the cannon fodder of your army.
If you want your spearmen to stand and fight, get some Yari Samurai instead - this way, you get
trained units who actually know what they're doing. Even better, get Katana Samurai; frankly, spears are
not designed for melee against infantry, and katana units will chop through them with ease.
In other news... how the bleedin' heck do you assault castles? I've recently discovered that charging directly at their walls with infantry isn't as smart a move as I originally thought (who would've thought?
), so what
is the trick? Should I start investing in siege engines? Lay siege and wait for them to sally forth to meet me? I swear, sieges were so much easier in E:TW, where you just lined up your cannons, blew apart the walls (while instakilling any infantry atop them), and then snipe their moronic cavalry. Shogun 2 has a nasty combination of competent AI and less-lopsided mechanics.