Тhe Tuida Fortress is a Late Roman, Early Byzantine and medieval Bulgarian fortress located on the Hisarlaka Hill in the eastern Bulgarian city of Sliven.
It was first excavated in 1982 by archaeologists from the Sliven Regional Museum of History and the National Institute and Museum of Archaeology of the Bulgarian Academy of Sciences. The archaeologists have discovered there remains of a Late Iron Age Ancient Thracian settlement (6th-1st century BC) which in the Roman period turned into a market place; a 2nd-4th century Thracian settlement is cited in written sources as Tuida, Suida, or Tsoida. The name is believed to be of Thracian origin, though its precise ethymology is still unclear.
The Tuida Fortress was built after the capital of the Roman Empire was moved from Rome to Constantinople in 325 AD. It is known to have had a secret tunnel built in the 6th century AD leading to the Novoselska River located to the west, a tributary of the Tundzha River.
The Tuida Fortress avoided destruction during the invasion of the Goths in 378 AD but was destroyed in the invasions of the Huns in the 5th century AD. It was rebuilt during the reign of Roman Emperor Anastasius I (r. 491-518 AD), preserving but also enhancing the original architecture of the fortress. The Tuida Fortress was ultimately destroyed around 598-599 AD, most probably during an invasion of Avars and Slavs.
The territory around today’s Bulgarian city of Sliven was made part of Bulgaria, i.e. of the First Bulgarian Empire (632/680-1018 AD), around 705 AD when Bulgarian Khan (or Kanas) Tervel gained the Zagore Region south of the Balkan Mountains after he helped Byzantine Emperor Justinian II the Slit-nosed (Rhinotmetos or Rhinotmetus) (r. 685-695 and 705-711 AD) regain his throne in Constantinople. Thus, a Bulgarian settlement, whose name remains unknown, was built on the place of the Tuida Fortress. A lead seal of Bulgarian Knyaz Boris I Mihail (r. 852-889 AD) has been found there.
The Bulgarians rebuilt the fortress walls and the aquaduct of Tuida, and erected new buildings inside the fortress that were covered with marble slabs produced by stone cutters in the then Bulgarian capital Veliki Preslav (“Great Preslav”). Several bricks with an Ancient Bulgar sign (resembling “|Y|") have been found there. Written sources indicated that Tuida was the seat of a bishop from the 4th century AD onwards.
After the original excavations of the Tuida Fortress first started in 1982, they were resumed in 2004. The archaeological finds there include, in addition to Knyaz Boris I’s lead seal, a number of iron tools, ceramic vessels, ornaments, coins, and bones from 14 species of wild and domesticated birds including Bonelli’s eagle (Aquila fasciata), western capercaillie (Tetrao urogallus), great bustard (Otis tarda), and common pheasant (Phasianus colchicus).
The archaeological excavations have revealed the fortress walls of Tuida, fortress towers and gates, remains of buildings, two marble pedestals dedicated to gods Apollo and Zeus which contain the name of the fortress as Tuida or Suida (known in written sources as Tsoida), a 3rd century AD inscription describing the settlement as a market place, a cult complex used between the 4th and the 13th century consisting of a three-nave one-apse Early Christian basilica and a unique baptistery decorated with murals and mosaics.
The ruins of a larger basilica have been found outside the fortress walls (which encompassed an area of about 40 decares (app. 10 acres)) which is taken to mean that the settlement was not confined by the fortified area but spanned outside of it.