The medieval fortress of Hisarlyk is located on the highest point of the hill of Hisarlyk, 2 kilometers southeast of Kyustendil. It was built at the turn of the 4th-5th centuries, and destroyed during the Ottoman conquest in the 15th century. The fortress wall had a width of 1.6 to 3 meters. The estimated height was 10 meters, and the height of the towers - 12 meters. During the construction, mixed masonry was used - rows of bricks alternated with rows of stones. To date, only a few remains of the structures are visible. Nevertheless, Hisarlyk Fortress is a cultural monument of national importance.