The last private owners of Mir Castle were members of the Svyatopolk-Mirsky family, who purchased these lands from the Wittgensteins in 1891.

The new owner of the castle did not restore the monument: for his residence, he built a new palace on the eastern side of the castle, which burned down in 1917. The palace was constructed in the forms of neoclassicism: a two-story stone manor house, with agricultural buildings nearby, and between the new palace and the castle — an English-style landscape park. In 1898, a new pond was dug in place of overgrown water bodies, for which the bastion fortifications on that side were dismantled. At the same time, the apple orchard growing there was mercilessly cut down right during its blooming period. A family chapel-tomb of the Svyatopolk-Mirsky was built in the park. Nikolai Ivanovich Svyatopolk-Mirsky died here at his Zamirye estate on the night of July 14 to 15, 1898, at the age of 65. The prince’s estates initially passed to his wife, Princess Cleopatra Mikhailovna, and later to their son Mikhail.

There is even a sad story that the founder of the palace died of a heart attack while watching the construction from a small island (this happened in 1898).

The Svyatopolk-Mirsky estate existed for a very short time. The house burned down at the end of World War I; in a 1917 photograph, the buildings are seen completely burned out.

The palace was two stories, executed in the classical architectural style. Besides the ruins of the basement, the surviving wing serves as a reminder of Nikolai’s former palace. Mikhail Nikolaevich Svyatopolk-Mirsky, who had previously served at the Russian embassy in London, decided to restore not the new palace that burned down in 1917, but the old castle. Restoration work on the castle began in 1922 and lasted 16 years. Between 1922 and 1929, the southwest tower was completely restored, along with the masonry of the walls of the eastern palace wing. After the economic depression, from 1934 to 1939, five spans of the eastern palace wing and two towers—the southeast and part of the northwest—were restored. So little is known about the Svyatopolk-Mirsky palace in Mir; there are not even any surviving photographs that would show this architectural monument. In 2017, excavations and studies of the estate’s basements were conducted. Perhaps, over time, new interesting facts about this place will be uncovered. If you visit the town of Mir, I recommend taking a walk through the landscape park along the pond and visiting the site where the palace once stood.

Sources:

https://ru.wikipedia.org/wiki/Мирский_замок