Throughout history, Korea's defense strategy against foreign invasion consisted in clearing the field and entering the fortress. To be more specific, Korean ancestors first removed all crops from the field and entered the fortress. They then waited until the enemies grew tired and began to withdraw. It was at that moment that Koreans started attacking them. Considering such a traditional policy of "clearing the field and entering the fortress," construction of Yeonhae Walled City meant searching for a new defense strategy. That was because it was lamentable to lose people, livestock and crops that had still remained in the field.
For a walled city, a variety of fortresses were constructed and the examples are stone fortress, earthen fortress, walled fortress and fortress composed of a wooden barricade and mud. During the Goryeo Dynasty, earthen fortresses were the most common. On the other hand, at the beginning of the Joseon Dynasty, both earth and stone were used and the inner part of the incline in the fortress body was built according to the construction technique used for the capital. However, during the 20th year of King Sejong, the Korean government published The New Guidelines on Fortress Construction. These guidelines were mainly about recommending masonry for walled cities. For example, all fortress bodies had to be built with stone and the inner parts of inclines made of earth and sand had to have a staircase made of stone. In the same year, King Sejong ordered construction of major defense facilities, providing guidelines on outer wall, outwork and storage. Stone fortresses came to be built following Choi Yun-deok's suggestion in March of the 16th year of King Sejong. At that time, Choi constantly argued for constructing stone fortresses. To elaborate on his position, he pointed out that during the Goryeo Dynasty, all fortresses had been made of earth and that it had been just a waste of human resources because these fortresses had been useless. He also added that since stone is everywhere in Korea, it must be used to construct fortresses so that they can be used permanently.
Following the advice of Hwang Bo-in, then Minister of War, The New Guidelines on Fortress Construction were published and stone became the official material of walled cities. The guidelines were drawn up by an organization called Sujseongjeonseonsaek.
However, using only stone for walled cities resulted in collapse shortly after the completion of a fortress. That is why during the 25th year of King Sejong, Lee Bo-heum asked the government to respect again the construction technique used for the capital to construct fortress bodies.
Archaeological research allows these changes to be observed in the process of extending Ungcheon Walled City. Such research also makes clear that after the first year of King Danjong, the construction technique of the capital was used again. In other words, before this period, a stone staircase was built on the inner wall of a fortress while later on, an incline made of dense earth and sand was used instead. This explains why during excavation, it is important to study how the foundation of a fortress body was built and to see if construction and reconstruction were carried out.