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Tzoida or Thuida - early Byzantine fortress is located on the Hissarluka hill on the northern edge of Sliven. It provides an excellent view of the "Blue Rocks" and the town. The fortress is part of Stara Planina fortification system, which has been an important role in defending the Roman Empire.
During the medieval period, the fortress was renamed Stilbnos (in Greek) and Sliven (in Bulgarian).
Kamen Stanev, “Kăde e bil srednovekovnijat grad Vlisnos (Blisnos, Blizmos)?” Minalo 4/2010: 19–27.
http://archaeologyinbulgaria.com/background-infonotes/fortresses-cities/tuida-fortress-sliven/