The lands within Sliven oblast have a long history and are object of many historical and archaeological researches which provide evidence for the existence of settlements in these lands, dating as far back as prehistoric times. These lands were inhabited by Thracians, proto-Bulgarians, Slavs, and later became part of the Roman and Byzantine empires. The rich ancient history left its cultural imprints. Some of the settlements, which were established in the region, declined, whereas others survived through the centuries and developed as important centres of culture and trade.
The name of the town of Sliven (Iztlifanost) was mentioned for the first time in 1153 by the Arabian traveller Al Idrisi who described it as a large and important town. However, the town itself must have existed since earlier times; probably it grew in the 3rd century A.D as a small Thracian settlement, named “Tuida”. Over the centuries the town gradually began to spread and turned into a spiritual, cultural and economic centre. Under the Ottoman rule the haidouk (i.e. revolutionary) movement was very active and strong here and the town became consequently renowned as “The town of the Hundred Voivodes” (i.e. a leader of a revolutionary detachment). Another proof of the active and enterprising spirit of the citizens is the first factory on the Balkan Peninsula, founded by Dobri Zhelyazkov in 1836, which determined the future development of Sliven.
Historically, the area of Nova Zagora is famous most of all for its rich archaeological finds of tumuli, necropolises, prehistoric settlements, tools, ornaments, some of which date back to the end of the 7th millennium and the beginning of the 6th millennium B.C. They provide valuable information on the manner of living of the ancient inhabitants. The unique Karanovo village tumulus, which had been inhabited for a period of 3000 years, is of great significance; remnants from all prehistoric epochs – from the Neolithic to the Bronze Age inclusive, are to be found there. Of equal historical importance is the area of Kotel, whose settlements Kotel, Medven, Zheravna became the cradle of the Bulgarian National Revival in all its strands – the national liberation, the church and the enlightenment movements; moreover, they took active part in the April Uprising and the Russian-Turkish Liberation War. A number of distinguished persons like Georgi Sava Rakovski, Zahari Stoyanov, Georgi Mamarchev, Neofit Bozveli, Sofroniy Vrachanski, Petar Beron, Sava Filaretov and many others matured both spiritually and intellectually here.
The town of Tvarditsa, which sprang out of a fortress, named Gradishte, has its contribution to the diverse and rich history of the district. The fortress with its geo-strategic location played an important role in the defensive system of the Second Bulgarian Kingdom. The name of the town has not been changed since its foundation.