The lands of Cathcart belonged to a family of the same name from the twelfth century. In the mid-fifteenth century the head of the family was raised to the peerage as Lord Cathcart, and the rectangular tower that gave the castle its dominant shape was probably built then. The Cathcarts held it for less than a century. In 1546 the property passed to the Semple family, a more powerful clan in Renfrewshire and beyond. The Semples added their own marks to the building; in archaeological work at the site, investigators uncovered fragments of a plaster armorial dating from the 1630s bearing the arms of Bryce Semple and his wife Jean Lauder of Hatton, the kind of ornament that would have sat above a hall fireplace, declaring whose roof you were standing under.

Local tradition long held that Mary, Queen of Scots, spent the night before the Battle of Langside in May 1568 at Cathcart Castle, and that she watched the battle the next morning from a nearby hill called the Court Knowe. The story is moving. It is also, modern historians agree, very unlikely. The Semples were on the side of Mary's enemies; they would have arrested her, not housed her. Court Knowe is fifteen metres lower than Langside village, so Mary could not have seen the fighting from there even if she had wanted to. A different tradition placed Mary instead with her kinsfolk the Stuarts of Castlemilk, in a room later known as Queen Mary's Room. That account collapses on the same kind of detail: Mary spent the preceding week in Hamilton, where her army of six thousand men gathered. The truth is duller and sadder. Her army was destroyed at Langside in less than an hour. Mary fled south, eventually to England and her cousin Elizabeth's prison. None of the buildings around Glasgow that claim her last Scottish nights can be confirmed. People want a piece of her ground.

The Semples built a new house, Cathcart House, on a nearby site in 1740 and moved out of the castle. The castle was already old, and once empty it began to lose stone to the surrounding farmsteads, the standard fate of unloved medieval buildings. The newly created 1st Earl Cathcart bought his ancestral castle back in 1814, with no intention of restoring it. He meant to sell the stone. Somehow, even that did not finish it. In 1866 the structure was still five stories tall, surrounded by outbuildings, a recognisable castle. Glasgow City Council bought the surrounding land in 1927 and added it to Linn Park, the green corridor along the White Cart Water on the south side of the city. Half a century later, the council decided the remaining ruins were unsafe, and in 1980 they were pulled down. The bookshelves of Glasgow Public Libraries still contain photographs of the standing castle. The grass in Linn Park contains only the marks of the foundations.

Archaeology and the surviving drawings let you reconstruct what stood here. The tower house was about fifteen and a half metres long, five stories including a vaulted basement, set inside a curtain wall enclosing a small courtyard, or barmkin, roughly twenty-two metres across. The curtain wall had round corner towers and a twin-towered gatehouse on its east side. Earthworks to the west have been read as the remains of an outer bailey or ringwork, the older fortified ground that this castle replaced. Stand in the right spot today, and you can still trace the shape of the building under your feet, the way you can sometimes feel the outline of a room in a house that has been gutted. Linn Park is one of Glasgow's loveliest green spaces, a deep slot of trees and water cutting through the south side of the city. The castle's absence is, in its way, also part of what the park is now.