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Local name: Крепост Хисарлъка
The mighty fortress was built on the highest, flat part of Hisarlaka Hill at the turn of the 4th and 5th centuries. It was rebuilt several times in the following centuries, until the 15th century, when it was destroyed by the Ottoman invasion. It has been in a ruin ever since. The fortress has the status of a monument of national importance.
The fortress is in the shape of an irregular polygon, 117 meters wide and 175 meters long. It was protected by walls 10 m high and 1.6 to 3 m thick. It was also defended by 14 towers, approx. 12 m high. Interestingly, they are not homogeneous - you can see the remains of the towers, both round and rectangular. Two gates and five secret entrances led to the inside of the fortress.
The walls of the fortress were built with a characteristic old Roman technique called opus mixtum. They were erected using alternating stone and brick, while using powdered brick mortar.