Like, in a certain way, you were far, usually a lot of archers, and have a weapon that can kill at a far good distance.

How actually usefull where archers? Did every army had a section of archers? How would they deal with flanking cavalary? Were shield enought to proctect against archers?

I understand their usefull in sieges, but i'm focus on field battles

I've heard of the english longbowman but would like a bit of more information on the matter