Constantine II

All that remains from prehistoric inhabitants on this area are small statues of idols and tools. Indigenous Illyrian tribes were conquered in the 4th century by the Celts. Celts ethically and culturally mixed with Illyric tribes and established on the right bank of the river Kupa a settlement called Segestica. Illyric and Celtic tribes succeeded in withstanding Roman pressures until the year 35 BC when Emperor Octavian with 12,000 soldiers conquered Segestica after a thirty - day siege. 

After Romans had conquered Segestica, they built Siscia on the left bank of the river Kupa (right below the centre of today's Sisak). Siscia was the capital town of the Province of Pannonia Savia, where 40,000 inhabitants resided. The town had the forum, basilicas, temples, an empire mint, a theatre and two ports.
Christianity was spreading unstoppably and encompassed the town of Sisak. The first known Bishop of Sisak was Kvirin from 284 AD until his martyr's death, probably in the year 303 AD.
With gradual collapse of the Roman Empire, the importance of Sisak declined and the great migration brought to Sisak Huns, Gauls, Avars and Slavs. Slav tribes remained in this area and eventually the Slav language became dominant. 

RIC VII Siscia 253 R3

Collection : LRB - Soldiers and Standard