It is understood that this is because castle construction using stone walls was developed in Saigoku which was culturally advanced area at that time and in Togoku, it was difficult to obtain stone materials which were suitable for castle building and therefore, dorui (earthen walls) were often used.
これは、当時の文化先進地である西国において石垣を多用した築城が発達し、また東国においては、築城に適した石材の採取が困難であり、土塁の使用が多かったためとされる。