"산성" refers to a mountain fortress or castle built for defense in Korea. These structures were often constructed on elevated terrain, providing a strategic advantage against invaders. They typically feature thick stone walls and watchtowers, allowing for better surveillance of the surrounding area. Many 산성 were built during the Three Kingdoms period and later, serving as military strongholds. Some notable examples include Namhan Sanseong and Hyangsan Fortress. Today, these historical sites are important cultural heritage locations, attracting visitors interested in Korea's history and architecture.