Imjonseong Fortress (예산 임존성)

Yesan Imjonseong is a Baekje-era mountain fortress on Bongsusan in Yesan-gun, with stone walls and an outlook that played a documented role in Baekje's last revival movement after the kingdom's 660 fall. The remaining wall sections are weathered but readable, and the climb to the summit takes about an hour through pine and oak forest. The view at the top — across the broad Yesan plain toward the Charyeong Mountains — is the same panorama that made the site militarily valuable. Quiet enough that you'll often have the summit to yourself on a weekday. Pair with the nearby Sudeoksa Temple, one of Korea's most important Buddhist monasteries.