Sisak is one of the oldest cities in Croatia with continuity of settlement. It's names changed from the times of Celtic Segestica, later Roman Siscia, then Kaikavian Sisek from 8. do 19. ct. to finally today's Sisak.
Sisek was a capital of a country called in historic sources Lower Panonia, ruled by prince Ljudevit, who was very well known to Franks. Lower Pannonia encompassed the area of later Zagreb Bishopric (also coressponds to kingdom Slavonia in Middle Ages}, meaning the Kaikavian language area. According to linguists Kaikavian language was differentiated from Proto-Slavonic already at the times of Ljudevit, so we can regard Sisek as 1st capital of 1st Kaikavian state.
The old fortress Stari grad was built in the middle of 16th. ct. Today it is a museum. For details on visit please check the upper link or call them on the phone.