This study is on the mountain fortresses at Cheonan area and a typical complex-type Fortress Sasan mountain fortress at Cheonan area in the period of Baekje. Cheonan is the place where Cheonan-gun, Jiksan-hyeon, and Mokcheon-hyeon were integrated and it has had a historical meaning since the period of the Three States. This place belonged to Mahan's territory during the period of the Three States and the area of Jiksan-hyeon is also presumed to be Mokji-guk. Jiksan has handed down the appellation of Wiryoe Fortress which is known as the first seat of Baekje Kingdom, and even topographically, it assumes a form similar to Seoul district. Jiksan was once the territory of Baekje in early period of the Three States, but it was integrated as Sasan-hyeon after Goguryo's occupancy, and after Silla's occupancy, it was integrated into Baekseong-gun as well. This place was a vital base for both Baekje and Silla. Baekje was in definite need of Jiksan in order to expand their influence into the Han river basin beyond the Anseong stream while at the position of Silla, Jiksan was the place which they had to definitely secure in an effort to defend Danghang Fortress and the Han river basin, both of which were trade routes with the Tang Dynasty. Sasan Fortress is a mountain fortress located at Jiksan―a strategic location as mentioned above―during the Three States. Sasan Fortress is a typical complex-type one in which valley-encompassing mountain fortress was prolonged at the outskirts of mountain-top-type or hillfort type mountain fortress.