The Ostroh State Historical and Cultural Reserve includes 11 architectural and historical monuments (9 monuments of national significance and 2 of local significance). 2 of the monuments are open-air museums, with the National History Museum and the Book and Printing Museum operating in their premises. 4 monuments are used for sightseeing, 3 monuments are religious buildings given for free use to religious communities. The structural unit of the reserve is a scientific library.
Particularly noteworthy is the Ostrogski Castle (based on the Donjon Tower), the oldest building in Ostrog, an architectural monument of national importance dating from the 14th century. The tower was built by the first known prince of Ostroh, Danylo, in the mid-14th century. A donjon tower is a type of single-tower fortress that combined residential and defensive functions. The donjon of Ostroh Castle is still the only well-preserved building of its type in Ukraine. In its more than 600-year history, the castle has had various owners, including the well-known princely families of Ostrogski, Zalavski, Lubomirski, Sanguszko, and for 108 years one of the largest and oldest museums in Volyn - the Ostroh Museum of Local History - has been operating in the castle.

https://ostrohcastle.com.ua/