The central location in the ensemble of buildings of the Castle Hill is occupied by the Church of the Epiphany. Researchers date the temple to the middle of the XVth century – first half of the XVIth century, and on the north wall of the temple, which is almost 3 m thick, there is an inscription with the date “1521”. Most likely, this is the year of the foundation of the brick church on the Castle Hill by the Grand Hetman of Lithuania Konstantin Ivanovich Ostrozky.
The northern wall of the Epiphany Church was included in the system of the castle wall, it still has loopholes, which visitors can see by going around the temple in a circle. The Church of the Epiphany also served as an archive, library, and most importantly – the family tomb of the princely family of Ostroh. In the crypts of the church are buried: probably Vasily Krasny Ostrozky; Halshka Ostrozka (1582); Alexander Ostrozky (1604) and his father Vasily Konstantin Ostrozky (1608).
After the tragic events of the Easter days of 1636, when the uprising of the Orthodox burghers of Ostroh was brutally suppressed by Anna Aloiza Khodkevich, the Castle Church was closed. For almost 300 years it was constantly destroyed, the reconstruction of the temple began only at the end of the XIX century. However, only the northern wall of the building survived, and the rest were dismantled to the foundation. A new brick church was built on it, preserving the three-dimensional solution and the structure of the plan of the previous church. Elements of architectural decor of hewn stone entered the Epiphany Cathedral in the form of inserts. Since the early 1990s, the church has been transferred to the reserve for use by the local community of the Ukrainian Orthodox Church