A substantial baronial mansion and estate in Annandale, Dumfries and Galloway, Castlemilk lies on the Water of Milk, 3 miles (5 km) south of Lockerbie. It was built 1864-70 on the site of an older property (dating from 1796) for Robert Jardine (1825 - 1905), nephew of William Jardine (1784 - 1843) the opium trader who founded the Jardine Matheson Company in Hong Kong. It represents one of the largest houses designed by David Bryce (1803-76). Castlemilk rises to three storeys and features pepperpot turrets, corbelled bartizans, pedimented dormer heads and crow-stepped gables, together with a substantial four-storey round tower and castellated porte-cochère on the east front. The house has been A-listed since 1971. The principal block, which forms the core of the house, was intended for entertainment and comprises a gallery that gives access to a dining room, library, drawing room and a conservatory, later remodelled as a Billiard Room. Said to be Bryce's finest work, the interior is richly-decorated Neo-Jacobean, making extensive use of oak in panelling, the staircase, book-cases and chimney-pieces. To the southeast of the principal block are private apartments, while to the northeast is the entrance and a service block.
A stable block, coach house and estate office forms a U-plan extension to the northeast of the house. The fine gardens include a parterre to the south of the house and a terraced garden which leads to a formal avenue of trees and reflecting pool to the west. To the north is a fountain also by Bryce which dates from 1870.