Kremenets Castle, Medieval fortress on Bona Hill in Kremenets, Ukraine

Kremenets Castle stands on Mount Bona, a limestone hill 397 m above sea level and about 100 m above the town of Kremenets. Its surviving parts are a square two-tier gate tower, a pointed Gothic entry arch, a tower known as Over the New House, and side walls 2.3 m thick and 8–12 m high. The inner bailey measured 65 by 135 m.

A hilltop fortress on Mount Bona (397 m) above Kremenets, built of limestone. It withstood attacks by Andrew II in 1226 and Batu Khan in 1240–1241, and was one of the few castles to survive the Mongol invasion. In 1261 it was demolished on the order of Prince Vasylko at Burundai's demand, then rebuilt in the 14th century by Liubartas, Vytautas and Svitrigaila. In 1648, after a six-week siege, Maksym Kryvonis's Cossacks took it; it was never restored.

The castle's name lives on in the town it overlooks, Kremenets, and in the hill called Bona after Queen Bona Sforza, who received the fortress in 1569 and had it rebuilt in Renaissance style. A memorial plaque on the walls commemorates the 1240–1241 battle against Batu Khan's hordes. Legend holds that Cossacks killed in the 1648 storming were buried in the Pyatnytsia cemetery below.

Access is via marked paths leading from the town center, with the ruins freely available to explore across the hillside. The elevated position offers views over the town and surrounding countryside.

The castle's unfinished well was begun in the 16th century but never completed, so defenders relied on stored water and gunpowder during sieges. On 23 August 2021 a 35-metre flagpole was raised on the hilltop, at 397 metres above sea level, and entered in the Book of Records of Ukraine.