Hisarlaka fortress is located on hill with the same name in southern part of Kyustendil, overlooking the town. It was built in the end of IV - beginning of V-th century when the town was mentioned under its Slavic name - Velbazhd. After this there were several reconstruction and extensions through the First and Second Bulgarian Kingdom and it became a major religious and administrative centre. The fortress was demolished by the Ottoman invaders in the XV-th century.
The fortress covers an area of approximately 2000 m2 and has a shape of an irregular polygon with dimensions of 117x175 meters, following the configuration of the terrain. It has 14 round, triangular and rectangular towers, two gates and five armies. The thickness of the wall varied from 1.60 to 3. 00 m and their height was about 10 meters (for walls) and 12 meters (for towers). Masonry is stone blocks cemented by the mortar with crushed bricks.
Hisarlaka fortress was declared a cultural monument of national importance.