Well, if you start as a Nord, you don't start mounted I believe. They have no cavalry by default; while you can train your own riding skill and purchase one, i don't think you'll start with one.
Oops, forgot about this.
Also me memorizing the troop class trees because I wanted to mod one day (and in a way, did), have the knowledge going:
> Khergits are mounted for forever. Even the new hires from a village have a ~50% to be mounted. However this means all those bandits are mounted too; ensure that if you travel into purple territory, you have a nice shield wall, or can do mounted battles yourself (preferably in a 2:1, skillwise; I gained a headache the first time I newb'd out due to all the spinning on horseback <_<)
> Nords have NO MOUNTS whatsoever. This includes their bandits, being the most skilled out of all, however all are unmounted. Sea Raiders, the toughest from Mountain Bandits, then Forest/Desert bandits. Khergit bandits are a whole different bunch due to their mounted nature.
> Rhodoks have great ranged capabilities and pride defense, but are meager in other stats compared to Nords and the other neighbors (each area does specialize in a certain theme or another), and there was a comparison to Swadia but this is the bit I forgot. One was a spear focused country. <_> But I do recall Swadia was a Bow focused one whereas Rhodok is a Crossbow focused one. [DPS versus Accuracy and Damage]
> Sarranids have a mix of mounted and otherwise, depending on where you generally are in the desert. What they also do have, is numbers.
From what little I remember of M&B, Rhodok castles are ridiculously defensible. In some of the setups the walls are a giant one way circle with no inner crenelations; once you get on the walls its a shooting gallery for the Rhodok troops. And those guys are already arguably the best in the game at siege warfare.
Nords also have a bonus to when they use shields, as my hazy memory recalls. While none of them are mounted, they are the best foot infantry, defense OR offense wise (Ok, technically their archers are mediocre in comparison to those specialized forces from Rhodok/Swadia (Rhodok = Crossbow; Swadia = Archer)), but their infantry are top-notch.