Rhodoks are full defence. They're the hardest faction to seige because of their heavily armoured units and the fact their infantry mainly use polearms and shields.
In a field battle you'll want to stand still and let the enemy come to you so they get cut down, trying to lead a charge rarely works for the Rhodoks.
If you're trying to take a castle with the Rhodoks it's best to order them to hold their ground and stop the ranged units from firing. That way the ranged units of the defenders should spend all their ammo on your shields and you can then lead the attack with ranged advantage.
So each faction have their own little niche which you can exploit,
Rhodoks - Defence, best for holding up in castles. Slowest in the overland map.
Nords - Assualt, best for taking castles. Decent in field battles. Lack of horsemen and decent archers will limit your tactics a bit.
Swadians/Sarranids - Heavy horsemen charge, best for running down infantry in field battles. All-round balanced units.
Khergits - Mobility, fastest overmap speed. Good on the bigger flatter maps if the horse archers don't get stuck on each other. Terrible at seiges, both attacking and defending. Try to "lead" seigers away from your castles so you can enagage them in a field battle. Make for the best "freelancer/bandit" playthrough.
Vaegirs - Archers and light horsemen. Never really found them all that impressive in vanillia Warband. Good if you stick some archers on some hills and then flank with horsemen. Decent for holding up in castles but your lack of good infantry means you'll have trouble keeping the attackers from your archer lines.