With the accumulations of outcomes from archaeological excavations of mountain fortress of three kingdoms period, there have been studies about time-periodic territory range of mountain fortress, difference in the way(method) of construction, defence system and so on from various points of view. This is an empirical study on the construction method of the valley part of stone fortress. First of all, it is required to secure large quantity of fresh water for those who lived at mountain fortress. Especially when builders of fortress construct a fortification at the valley part of stone fortress, in advance they must sufficiently consider several options including the establishment of sustainable water resources. First, when it comes to build a fortification on a ridge[or a slope] of a mountain, you have only to consider a vertical stress. However, when it comes to build a fortification at the valley part of a mountain, You must have more sufficient preparations for the constructing process. Because there are not only a vertical stress but also a horizontal pressure simultaneously. Second, a fortification of mountain fortress built by using unit building stone is a structure of masonry construction like brick construction, and the valley part of it is where the construction of the fortification begins. Third, when it comes to build a fortification at the valley part of a mountain, it seems that they use a temporary method such as coffer dam in oder to prevent the collapse of the fortification due to heavy rain. Furthermore, in response to a horizontal pressure a fortification is built by the way of its plane make an arch, or by piling up the soil with the plate method(類似版築) and earthen wall harder method(敷葉) they increase cross-sectional area of the fortification and its cutoff capacity. In front direction they put the reservoir facility for the fear that the hydraulic pressure and earth pressure are directly transmitted to the fortification. The process of constructing the fortification at the valley part of a mountain is done in the same oder as follows; leveling of ground(整地) ⇨ construction of coffer dam ⇨ construction of the fortification between the both banks of the valley ⇨ construction of the fortification at bottom part of spill way(餘水路) between the both banks of the valley ⇨ construction of spill way(餘水路) & reservoir facility ⇨ construction of the fortification at upper part of spill way between the both banks of the valley. Coffer dam facility seems to be not only the protection device on occasion of flood but also an important criterion to measure the proper height of spill way or tailrace(放水路). This study has a meaningful significance in that it empirically examines the method of reduction of the horizontal pressure which the fortification at the valley part of a mountain takes, the date the construction was done, and wether the changes in climate such as heavy rainfall influence the process of construction.

신라왕조의 석축 산성에서 조사된 성벽을 관통하는 배수장치에 대한 기왕의 연구를 보충하였다. 조사된 사례가 3개소에서 12개소 이상으로 증가된 고고학적 발굴조사 성과를 반영하여 형식적 구분을 다시 하였다. 성곽에서의 배수장치로서의 수문은 교량형(橋梁形)과 암거형(暗渠形), 누조(漏槽) 등으로 크게 구분된다. 이글에서는 특히 성벽 통과형 암거형식을 다루고, 그 시설된 숫자에 따라 단구식(單口式)과 다구식(多口式)을 구분하였다. 다구식은 성내의 수위 변화에 대비하여 수위(水位)에 따른 배수가 가능하게 만든 것들이다. 종래 출수구(出水口) 외면의 형태에 따라 형태를 구분하였던 점을 고쳐서 입수구와 출수구를 잇는 수로의 횡단면 형태에 따라 오각형, 사다리꼴, 사각형에 추가하여 삼각형의 유형이 있음을 새로 추가하였다. 이중 가장 보편적인 사례는 방형 단면의 것들이며, 이런 형태는 신라 하대에서 고려를 거쳐 조선시대 후기까지도 그 사례를 찾을 수 있는 반면, 나머지 형태는 신라의 5세기 후반에서 7세기에 걸쳐 다양하게 축조되었다. 이들 여러 형태 가운데 단면 사각형은 중세와 근세의 산성까지 가장 오랫동안 시설되었다.