In the middle of the river Gacka.
Otočac owes its name and its survival to a river island. Up until the 18th century Otočac was situated on a small island on top of the Gacka River.
The island was surrounded by fortified walls and towers for protection, and could have been accessed by boat only. This fortress made it truly impregranble and throughout its long history the town Otočac has never been conquered by an outside force. Traces of early human habitation have been found locally. The hill forts in Umac and Vinica, and the nearby Zorišnjak near Staro Selo, testify to the settlements dating back to the Iapodic time. These hill forts are yet to be subjected to detailed archaeological study.
The oldest preserved deed of donation regarding Otočac dates back to 1300, whereby King Charles II of Naples donated land and the town of Otočac to Doimus II, the Prince of Krk. The princes of Krk, later called the Frankopans, had the town fully fortified, built churches and encouraged the rise of culture and literacy. Throughout their almost three-centuries-long reign over these lands, the princes Doimus II, Frederick III, Sigismund and Martin Frankopan have all at one point resided in Otočac. It was under Sigismund that Otočac saw a period of prosperity. On the 5th of March 1460, Pope Pius II founded the Otočac diocese and upgraded the collegiate church of St. Nicholas into a cathedral, with Otočac granted the civitas status by a Papal Charter.