on a VI note, Celtic Warriors with a decent general/valour upgrade will carve through almost anything the non-Viking factions can throw at you in shocking fashion, although they have no armour (only snazzy bi-coloured trousers) so need a lot of post-battle replacements.
paint 'em with woad, field 'em three deep, get the high ground and then throw back your shaggy hair (see Stephen Dillane as Merlin in 2004's King Arthur), yell something rude in Welsh and let 'em roll.

i agree with rvg, in MTW swordsmen, the heavier the better, are excellent for castle assaults. i prefer to use billmen on the battlefield, or halberdiers if i'm not English, mainly for flanking while Chiv/feudal sergeants do the hard work. i find they always take a lot of casualties though, however i use them.