Complex sentence: There are several city wall remains in the area dating from this time, and Pungnap Toseong, an earthen wall just outside Seoul, is widely believed to be the main Wiryeseong site.
Split into simpler sentences: There are several city wall remains in the area dating from this time. And Pungnap Toseong, an earthen wall just outside Seoul, is widely believed to be the main Wiryeseong site.