The medieval castle Ozalj is situated on a cliff above river Kupa. Castle was first mentioned in 1244. as a free royal city. From the late 14th Century (1398.) it was in possesion of Frankopan noble family, and in 1550. it becomes property of Zrinski family, due to a family relations of these two noble families.
After an unsuccessful plot against Habsburg rule, on 30. April 1671. Croatian nobles Petar Zrinski and Fran Krsto Frankopan were executed in Wiener Neustadt, under charges of high treason. This event is known in Croatian history as a 'Zrinski–Frankopan Conspiracy'. The castle was then robbed and abandoned.
The ruined castle changes several owners, and it was restored in period 1743. - 1753. Today the castle houses a museum which was founded in 1971.