In 1982 begin large-scale excavations in the area, during which an epigraphic monument with an inscription is discovered; it determines the ancient name of the settlement – Tuida. The inscription reads, „To the always listening Zeus Okonenski Julia Marcia and her husband Aurelius Theopomp, a municipal councillor, set with gratitude (this monument) in the market Tuida.“ According to the paleographic data the inscription is from the beginning of the 3rd century. From this inscription we find out that on the territory of the present day town Sliven existed a settlement – a market place (emporion), a center of agriculture and trade. In this market place existed a sanctuary with a tempel dedicated to Zeus Okonenski. From another inscription found during the excavations of the western wall, we learn the name of the settlement in its other form: Suida. Apollo was also worshipped in the sanctuary. In the interior of the fortress were found other sculptural monuments of a religious nature. Among them stands out the torso of a marble statue of the god healer Asclepius and votive tablets of a Thracian horseman. One of the votive tablets of the Thracian horseman has an inscription which reads:
[. Ο]ὐέρριος Οὐά[λης? ὑ]πὲ[ρ] [ἑαυτοῦ] καὶ τῶν ἰδίων [---] χαριστήριον α-ε---
“[-] Verrius Valent (or Var?) [dedicated/set] ... as a gift of gratitude for himself and his loved ones ...“
The person who made this dedication was a Roman citizen with three names, the first of which was written in an abbreviated form at the beginning of the line and is not preserved. The family name is the relatively rare name Verrius; the cognomen which begins with Va- is probably Valens or Varus and is also undoubtedly Roman. According to the form of the letters, the inscription can be dated in the 2nd century. These finds show that the Thracian settlement Tuida (Suida) was very well-built and economically developed. The hill in the Hisarlaka area was formed as a sacred complex.
In the middle of the 4th century the fortress is built in the Hisarlaka area in Sliven. During this period, in the fort is built the great basilica with a nave and two aisles.
In the middle of the 5th century the fortress was destroyed after Thrace was ravaged by the Huns. Life in the fortress died down for almost half a century.
At the end of the 5th and beginning of the 6th century the fortress was restored by the Emperor Anastasius. It is known that during the reign of this ruler begins a building boom of fortresses in the Balkan territories of the Empire. The fortress was built almost from the ground up, but its layout and type were preserved. To the erected wall were added additional facilities – stairs, sewers, water-mains. The interior of the fortress is built densely with buildings with various functions, dimensions, design and technologies. At that time was formed the old-Christian complex; the large destroyed basilica was restored and southeast of it rises an autonomous baptistery with a complex structure and colorful mosaic floor. Procopius of Caesarea in his work „The buildings of Justinian“ reports that at the time of Justinian I the walls of the town Tsoida were repaired. In the earliest diocesan lists, in the so-called Epiphaniev List, an Episcopal center Tsoida is mentioned which was subordinated to the Archbishopric of Adrianople. This statement is supported by the old Christian complex discovered in the fortress. The construction work continues outside the city walls. South of the fortress was built a large basilica with a nave and two aisles, which was used by the population outside the fortress. The settlement in its fortified part was relatively well developed. There was a well-built water and sewage system and large public buildings. A passage and a tunnel were also built, providing additional opportunities for a water supply from the river Novoselska. There is an economic recovery during this period expressed in a brisk circulation of coins, numerous finds of handicraft items and luxury.
At the end of the 6th, beginning of the 7th century the Avar-Slavic invasions of the Balkan peninsula put an end to the Early Byzantine city for nearly two centuries.
At the end of the 8th, beginning of the 9th century the hill was populated again. The new occupants do not comply with the old buildings of the fortress. Their homes are incised into the uncleared debris formed by the destruction of the fortified walls.
The walls of the old fortress probably were preserved to a considerable height; they were repaired and used accordingly. The old towers and gates were used, although their floor levels were raised; they were not cleared to their original floor level. Whole buildings and structures inside the fort remained under a thick layer of debris – the baptistery, the horreum, the administrative building, the building with the underground tunnel.
Over time, in the fortress was done further construction work related to its public development. From the north, in front of the north gate was constructed a water-main, and to the north and east wall were sticked stone staircases. Bricks were found with an engraved Old Bulgarian sign I Y I.
During the excavations was discovered a bulla belonging to Kniaz Boris Michael which is dated in the 870 AD and a marble facing plate with an embossed decoration plant produced most likely in one of the sculpture shops of Veliki Preslav for representative constructions.
Life in the fortress continued after Bulgaria fell under Byzantine rule.
By the middle of the eleventh century the nomadic Pechenegs, Cumans and Uzi pervaded northeastern Thrace. The Pechenegs captured the fortress and they ruled in it for some time. Until the mid-twelfth century the local Bulgarian population which left the fortress during the invasion of the Pechenegs returned and was settled in it again. From the life conditions of the people already was noticable an obvious decline. The walls of the fortress were repaired, but the construction technique was primitive and careless. Over the stone staircases were built ramshackle buildings of stone and mud. The old Bulgarian water-main was broken along its way by garbage pits. In the northeastern tower a home was built. The Tower of the south gate was destroyed, and the walls were cut by pits.
At the beginning of the thirteenth century life in the fortress in Sliven was interrupted forever. Within the boundaries of the old fortress after its abandonment continued to operate only the necropolis around a small cemetery church built on the embankment covering the monolithic early Byzantine administrative building.