Maybe I'm missing something, and I haven't been in a campaign for a long duration (up to 1180 AD I think), but I mostly sea either spear units, dismounted knights/nobles and the odd swordmen, like nordic swordmen, or axe wielders like Gallowglasses and vikings/huscarls. What about men-at-arms?
Will they come at a later satge in the game? but what invention is needed or do you need a very advanced building?