Tuida was an early Byzantine and medieval fortress, the remains of which are located on the hill of Hisarlaka in the northeastern part of Sliven, which offers a unique view of the 'Sinite kamani' and the city. It is part of the old mountain fortification system, which played an extremely important role in the defence of the Roman Empire, and later of the early Byzantine Empire, as well as the medieval Bulgarian state. Tuida Castle was originally built in the 4th century AD and was in use until the 13th century.
Duino Castle was built by the Wallsee family in 1389 on the cliffs overlooking the Gulf of Trieste. It replaced an older castle from the 11th century. Over time, the Wallsee family disappeared and the castle, after having been used as a prison, became the residence of the Luogar and Hofer.
At the end of the 19th century it became the property of Prince Alexander von Thurn und Taxis from the Czech branch of the House of Thurn and Taxis. It remains with the family to this day with his great-grandson Prince Carlo Alessandro della Torre e Tasso, Duke of Castel Duino the current owner. The castle has been opened to the public as a museum and park.