It is an original archaeological monument that dates back to the epoch of the late antiquity.     It is situated on the "Hisarlaka" hill to the north-east of Sliven where one can enjoy a great view of the magnificent Blue Stones and the town.      The fortress is part of the fortification system along the Balkan mountain. It played a very important role in the defence of the Roman empire, and later on in the defence of the medieval Bulgarian kingdom.

In 1982 there began systematic excavation works of the fortress, carried out by the Historical Museum of Sliven and the AIM at the Bulgarian Academy of Science.    Unique architectural monuments were discovered during the excavation work, such as the fortress walls, turrets, the North and South gates, the foundations of dwellings, outhouses and military premises, remnants of Slavonic dwellings.    The cult complex, discovered in the eastern part of the fortress, has an exceptional scientific value. It consists of an imposing basilica with a nave and three aisles and a unique six-walled baptisterium with a baptizing pool which was lavishly decorated with mural paintings and mosaic.    The stratigraphy of the monument and the rich findings give us good reason to date the fortress back to the middle of 4th C with its history going on till 13thC.    Of great scientific interest are the two epigraphic monuments found during excavation works.    There the name of the fortress is Twidda or Swidda. They gave us the opportunity to locate around Sliven the early Byzantine fortress Tsoida, mentioned in different written records.    Of extremely high value to science is the seal of Knyaz Boris I, together with the collection of iron tools, pottery, bone pieces and jewellery and the significant number of coins.    After a certain interruption the archaeological works at the fortress will begin again in 2004. Their aim is the final study and preservation of the architectural and archaeological monuments, the opening of the fortress to the public with the aim of turning it into one of the remarkable tourist attractions of the town and a recreation centre for the citizens of Sliven.    Even now lovers of ancient ruins can go and see the fortress.