The Proto-Rhodoks have very good (not the best, I think the Sturgians have better elite archers) archers and very good large shield units with javelins and spears, so they can counter any cavalry (and elite archers can handle a decent cav charge with minimal fuss)
However they suffer HEAVILY in sieges due to lack of heavy armor (I'd say they're medium heavy armor? like plate and mail versus heavy plate) but in an open field battle, they're nigh unstoppable. So how to conquer? Keep sieging until the enemy sallies out, it's always a field battle. Then you have two or three defenders left, you assault the castle and boom, you're golden