Hisarlaka is a late Antiquity and medieval fortress occupying a hill at 2 km to the south-east of Kyustendil. Originally constructed by the Romans, it was among the most important castles of the Bulgarian Empire in the Middle Ages before it was partially destroyed by the Ottomans in the 15th century.
The city walls of Hisarya date from the Roman Empire, when the settlement was known as Diocletianopolis. Hisarya has the best preserved Roman walls in Bulgaria and once of the best preserved in Europe. Their length is 2327 m, the width varies between 2.60 and 4 m, and the height reaches 12.5 m. There were 44 towers, 4 main gates and 6 smaller entrances. Two of the gates, the southern and the western, are well preserved.
