The city was heavily fortified during the 15th century as a result of its strategic position, and during the Joseon Dynasty, became a famous retreat for the aristocratic Yangban due to its fantastic scenery. The city's Yaksan, meaning medicine mountain, was well known for its beautiful azaleas; the famous modern poet Kim So-wol wrote one of his best-known poems on this subject. Two other mountains, Yaksan-dongdae, east of Yaksan, and Moran Hill are also famous scenic spots. Yongbyon also houses many important relics, including the Chonju and Soun Buddhist temples, dating from 1345 and 1684 respectively; the Ryuksung Pavilion, famous for "six scenic views" of Yongbyon; and the Cholong Castle, built to protect the city during the Yi dynasty. The south gate of Yongbyon, called Mannomun, is also famous.