It is supposed that a Thracian settlement appeared in the area during the III century AD.
The name is probably Thracian, but of unknown origin. The region Sliven became part of the First Bulgarian Empire about 705 as part of the Zagora region which was populated by Slavs and given to Tervel according to his contract with the Byzantine emperor Justinian II Rhinotmetos. In the place of Tuida arose a Bulgarian settlement with an unknown name.
It appeared before the year 870; this information is confirmed by the lead seal of Kniaz Boris I Michael which was discovered during archeological excavations. It is assumed that the fortress has been destroyed twice - once by the Huns before the rule of Justinian I, who restored it and a second time about 598-599 AD, when it was destroyed again, probably by Avars and Slavs. The Bulgarians repair the walls, even the water-conduit by the north gate.
New buildings were built in the interior, some of which are faced with marble tiles made in stonecraft workshops in Preslav. From that period were found several bricks engraved with the Bulgar sign Upsilon flanked by two hasta.