Barr Castle is a 15th-century keep which was altered in the 16th century (and probably later, too). The gables of the castle have collapsed, apart from the chimney stack which juts up into the sky. Not much survives of the former courtyard of the castle. The main hall was on the first floor, reached by a turnpike stair, which continued to the rooms on the upper floors. It is clearly visible from the A760 road going south from the village to Kilbirnie. It was built by the Glen family but passed in the late 16th century to the Hamiltons of Ferguslie. A door lintel has a date of 1680 and the initials L.H./I.C. The family abandoned it in the 18th century in favour of a new house.
The family, variously known as Sempill, Sempil, Sempel and Semple, had probably owned estates in the area from as early as the 13th century. Robert Semple, Steward of the barony of Renfrew during the reign of Alexander II, was recorded as living in Elliston Castle, whose ruins lie near Howwood. The Semples of Elliston fought for Robert the Bruce, and steadily grew in power to become the Steward's hereditary Baillies of Renfrewshire. They were appointed Hereditary Sheriffs of Renfrewshire and Hereditary Baillies of Paisley. They were later designated as Lords Semple. Their extensive land holdings, Castle Semple, constituted some areas of Lochwinnoch and its hinterland. At some point, probably in the 15th century, the family built a tower keep at the east end of the north shore of the Loch.