It's the first time I play SoD, but I have to say I'm loving Zerrikanians.
Their mix of ranged and melee cavalry, of fast and powerful horses seems to crush everything I've met so far.
Also, with their blunt weapons they take a lot of prisoners, which for me has translated into a ridicoulous amount of gold and some 1100 troops recruited into 5 regiments.
Then, around day 100, I've started my war against Khergits (first power in Calradia in this game).
They've already lost 2 castles in a day!
I'm using a strange mix of troops: I keep all my party mounted, with companions and Zerrikanian troops; the 5 regiments following me are composed of Forest Bandits, Bandits, Cutthroats, Sea Riders, Villainese Infantry and Longbowmen, and an handful of mixed Calradian infantry and missile troops.
So far, it seems that using a mass of expendable troops is a good strategy.
They are cheap, allowing you to field an horrendous amount of them; they have somewhat decent stats that allow them to actually cause some damage; and they distract the enemy. Every missile fired at one of them is a missile that is not killing one of my best troops spread out between them!
We'll see how it turns out in the end.
I'm trying to pull off a blitzkrieg, taking every Khergit castle and city as fast as I can, without even giving any of them to lords. Then I'll take some time to develop my new kingdom.