Kostomlaty Lower Castle, Cultural monument in Kostomlaty pod Milešovkou, Czech Republic.

Kostomlaty pod Milešovkou is a Baroque château built in 1670–1684, made up of two rectangular buildings facing the Church of St. Lawrence, the northern one set slightly forward. Cylindrical towers stand at both eastern corners, the south-east one holding a spiral staircase, and the two-storey seven-axis façade has arched first-floor windows and semicircular pediments on the ground floor. A three-wing northern section and a small L-shaped wing were added around 1900.

A medieval castle above the village was the lords' seat from the 14th century, but the estate was split among owners in the 16th century. Brothers Oldřich and Petr Kostomlatští z Vřesovic built two fortresses in the village, confiscated after the Bohemian Estates uprising; Humprecht Jan Černín z Chudenic had the Baroque château built on the site of one in 1670–1684. In 1888 it was converted into a reformatory for women.

The castle's name comes from the village, whose lords had their seat in a castle above the village from the 14th century. Since 1888 the buildings have served as a women's reformatory and later as a children's home with a school and secondary school, uses that continue today.

The château stands in the village of Kostomlaty pod Milešovkou, Teplice district, opposite the Church of St Lawrence. Since 1888 it has housed a correctional and later educational institution, so the interiors are not open to tourists; the two-storey Baroque façade with its cylindrical corner towers is viewed from the village street.

Renaissance masonry from an older fortress is thought to survive inside the central section of the Baroque castle, so part of the 17th-century building may stand on the walls of the earlier stronghold. The two cylindrical towers at the eastern corners of the seven-axis facade are another detail to look for.