The teutonic order in kingdoms has that. Can only get minor cities or lower and their castles were pretty awesome. But not so sure on having really low economy. The dwarfs are one of the richest civs in warhammer with the massive demand for dwarf made goods and all the mines. However they have a great reluctance to spend it on anything other than the nessecites. Maybe you could put in a market or trade route in castles but im not sure if that is possible. Possibly increasing the amount of gems/gold/iron/coal and the kings purse.