In January 1561, the small village of Kolishchyntsi became the property of Prince Vasyl Kostiantynovych Ostrozkyi. The prince acquired this settlement from Vasyl Labunskyi and, in three months, received the privileges of the Magdeburg right from the Polish king. In honor of his father, he named the new town Kostiantyniv. In 1632 the city was renamed Starokostiantyniv since another Kostiantyniv had already existed.
The town was located at the very frontier of the Volhynia possessions of the Ostroh princes, namely on the border with Podilia. From this side, the Tatars attacked Volhynia most often. Therefore, the settlement was expected to be a powerful fortress that would stand against the enemy's attacks.
Since then, Prince Vasyl began to call himself by a double name of Vasyl-Kostiantyn in order to inherit the glory of his father and to be associated with the Byzantine Emperor Kostiantyn as well as the town of Kostiantyniv with the Byzantine capital of Constantinople.
When Vasyl-Kostiantyn took over Ostrog again in 1574, the prince set about building and strengthening the capital of the Ostroh principality rather than improving Kostiantyniv. Nevertheless, he did not stop caring for Kostiantyniv. He went on developing the castle here, built five temples (one of which was located at the castle building), and founded a school there.
Of the five churches, two were stone and three were wooden. In the early 17th century, the prince began the construction of the Church of the Exaltation of the Holy Cross, which Yanush Ostrozkyi rebuilt into a Dominican church and monastery in 1613. The Church of the Exaltation was embodied in the defensive city structures as well as its 30-meter bell tower served as a defensive tower.
In 1620, the town came into the possession of the princes of Zaslavl. Later in 1682, it passed to the princes of Lubomyr.