The supplementary defensive castles(Chisung, 雉城) at a town castle located at a small town along the coast waters of the south sea were representative fortifications, which were built as a set of structures and a town castle's subsidiary facilities along with impregnable protective castle walls, castle gate grounds, moats, and lower defensive and offensive walls built on top of a castle(Yeojang, 女墻) during the Joseon Dynasty period, so those data is really important to understand the structure and construction methods of fortifications at the coastal towns of the south sea at that time.
In case of Gyeongsang-do, there existed town castles built in a location where provincial higher administrative units along with Daedohobu(a kind of local administration, 大都護府) and barracks at Gyeongju, Gimhae, and Changwon, and the town castles showed a difference in size from other town castles belonging to the lower administrative units by one point five to two times. These aspects show that the higher level the administrative units, the more supplementary defensive castles and besides, at the town castles having barracks and naval barracks, the much larger numbers of supplementary defensive castles in the town castles were planned and built in comparison with other town castles built at general counties or prefectures.
The basic numerical value of supplementary defensive castles in town castles was divided into 6-site-location and 12-site-location in the Joseon Dynasty period. In other words, the two types of supplementary defensive castles were as follows; the 6-site-location consisted of four fully-grown bulls and two right-left symmetrical castle forms at the main gate site ; the 12-site-location consisted of four fully grown bulls with two supplementary defensive castles respectively at the left and right of main gate site.
Therefore, the fact that more than 12 locations of supplementary defensive castles were built proves that the Joseon people fortified their defensive facilities to protect town castles from the Japanese pirates at that time who frequently committed provocative acts especially at the coastal town castles along the south sea; thus it is presumed that the Joseon people's fortifications project once centered on Hasam-do during the early Joseon period turned its most emphasis on the coastal areas of Gyeongsang-do, i.e. coastal town castles along the south sea.
Among the coastal town castles, inland town castles and supplementary defensive castles of Youngjin Castle(營鎭堡城) was the square type ground plan used for building for the longest time and the next was a rectangular-base with narrow flat upper section, and the last was a rectangular form. It is found that in the former times on King Sejong's 15th year ruling period(1433), there had been a lot more frequencies of a rectangular square with almost the same ratio of length over width, which was used either by Naesangseong where barracks and town administration commonly co-existed or by town castles built in the higher level unit towns among the administrative districts.
Next, from after the 15th century through the 16th century, the ground plan for the supplementary defensive castles was mainly a regular square and a rectangular; in contrast, rectangular base with narrow flat upper section diminished or extinguished. Likewise, in case of naval forces Youngjin Castle built before and after the 15th century, the regular square had been a main-stream, and such convention was passed through the early-middle Joseon Dynasty periods to the later period of Lee's Dynasty.
In addition, in King Sejong's 15th ruling period(1433), rules and forms for supplementary defensive castles in town castles were applied, and accordingly, rectangular base with narrow flat upper section form castles were built; however a square form and a rectangular form were also continuously used in building supplementary defensive castles.
Besides, in case of rectangular formed castles, it is found that it had been longer used than the rectangular base with narrow flat upper section form castles and also the standard for building rectangular base with narrow flat upper section form castles wasn't observed well at that time.
Up to this time, it has been presumed that the fact that supplementary defensive castles had various ground plans with different length and width in the same town castle was due to the difference in measurements and the length of periods for castle construction. However, supplementary defensive castles were built with difference in length between castles to serve mutually supplementary functions and that's why a square, a rectangular, and a rectangular-base-with-narrow-flatupper- section form were used together in building supplementary defensive castles of inland town castles in Youngnam District including coastal town castles along the south sea. Thus, it can be concluded that the supplementary defensive castle construction was done with width and length decided by its surrounding geological condition or castle circumference and in case of Youngjin Castle-the higher administrative and military command system, it had supplementary defensive castles built with a greater length and width.
Next, the five-meter length standard[which was the established rules and forms at that time] of the supplementary defensive castles were usually observed and they were built near the coastal town castles along the south sea.
In addition, such rules and forms of five-meter are found in the supplementary defensive castles of Youngjin Castle built during King Sungjong and Jungjong periods and the same with the supplementary defensive castles of later built Dongrae town castle; thus it can be said that the rules and forms of five-meter-long measurement had been observed continuously through the whole periods of Jeosun Dynasty.
Therefore, coastal town castle built along the south sea areas, as indicated in case of supplementary defensive castles, used a single construction pattern in their numbers and size of defensive facilities including supplementary defensive castles in comparison with the other districts' during the same periods, which fact proves that the coastal areas along the south sea, due to military function as a strategic stronghold against Japanese pirates invasion, were under strict castle-construction control from the government, thus consistently carrying out with castle-construction project.