i find the human alliance to be the most effective, with the right strategy, first off aim for the demons first, the attack the strongest countries to get them out of the way (preferably get rid of trolls and undead strong points first) and use spam swordsmen, use scouts on open areas while spamming swordsmen down one file, except to block enemy, and use kingsguard strictly for charge, upgrade only sword speed and armor and dont buy any units but scout and kingsguard and youll win easy
when using the normans, try to form a shield wall with your men at arms, and use cavalry only for charges, and when not in use, have them taunt from some small distance behind the shield wall
i find that using the human alliance, upping sword and armor, is a great way to make proggress, as for the demons, whenever one comes at you, spam the regular swordsmen at it, and for charges use the kingsguard
just a tip, the way to gain an max enchantment immediatelly, is to use the save and load feature to advantage, before refining the weapon, save, then crank the refine level to the max, and if it breaks, load and try again
i find tghe far east empire is easier to take over if youre one of them... always play as the ninjas, and conquer the horse tribes first. Id say the only downside is you need to buy a ship immediately