William Borthwick acquired most of the Mote of Locherwart from his neighbour William Hay, who - the chronicles say - resented selling and was jealous of the castle that would rise on land that had been his. The site itself was already old: there had been an earlier structure here, and the surrounding ridge gave protection on three sides, with the ground falling steeply away. The medieval effigies of William Borthwick and his wife can be seen today in the nearby parish kirk of St Kentigern, which keeps a 15th-century aisle probably also built by him. The castle is built as a double tower 74 feet long and 68 feet broad, with a great chimney that rises in elaborate decoration. Inside it had vaulted halls, private chambers, gardrobes, kitchens - everything a great medieval household needed. The shot-holes pierced at the corners of the outer wall remind you what it was actually for.

In September 1544 John, 5th Lord Borthwick was captured by George Douglas of Pittendreich and held at Dalkeith Castle. His wife, Isobel Lindsay, did not wait for diplomacy. She invited Patrick Hepburn, 3rd Earl of Bothwell, to visit Borthwick - and then imprisoned him. He was held until her husband was released in exchange. A spy reported back to the English officer William Eure that Bothwell had come to the castle because 'the Lady Borthwick is fair, he came to her for love, but she made him to be handled and kept'. Thomas Wharton heard that Bothwell had been invited at night to a newly-built lodging outside the castle, where Gavin Borthwick took him prisoner. These were not the manners of a quiet domestic life. They were moves in the power struggle of the Rough Wooing - the English king Henry VIII trying to force the marriage of his son Edward to the infant Mary, Queen of Scots, and the Scottish nobles choosing sides accordingly.

Mary, Queen of Scots came to Borthwick in August 1563 and again in October 1566. Then, on 15 May 1567, she married James Hepburn, 4th Earl of Bothwell - a controversial match almost everyone in Scotland opposed. Within weeks the new couple were under siege at Borthwick by Lord Home and the laird of Cessford. The escape became one of the most famous of Mary's many escapes: she disguised herself as a male page and slipped through the besiegers' lines, leaving the castle and her husband behind. Bothwell followed shortly after by a separate route. They reunited briefly, but the marriage was already finished. Mary was arrested soon afterwards and imprisoned at Lochleven Castle. Bothwell fled to Orkney, then to Norway. The Danish king kept him as a political pawn, first at Malmö and then at Dragsholm Castle on Zealand. He died there in 1578, having lost his mind after years of harsh confinement. His embalmed body is preserved in a crypt at Fårvejle church nearby. He never saw Scotland again.

In 1650 Oliver Cromwell's forces attacked the castle as part of the campaign that followed the Battle of Dunbar. Borthwick surrendered after only a few cannon shots - the defenders apparently recognising that resistance against modern artillery was futile - and the damage from those shots is still visible today in the eastern wall. After this episode the castle entered a long period of decline and abandonment. The Borthwick family eventually moved out, and the great tower sat empty for much of the 18th and 19th centuries. Restoration began in the early 20th century and was largely complete by 1914. During World War II the structure was used to store national treasures evacuated from places more likely to be bombed - the castle's thick walls and remote Midlothian location making it about as safe a hiding place as the country had.

In 1973 the Borthwick family leased the castle to operators who converted it into an exclusive private-hire venue. From 2013 to 2015 it closed for extensive refurbishment, reopening as an events venue in September 2015. Today, panoramic views of the castle can be seen from passengers on the Borders Railway between Edinburgh Waverley and Tweedbank stations, which runs through the valley below. The Borthwick family are still its owners after nearly six centuries - one of the longest unbroken family possessions in Scotland - though they no longer live in the keep. Local ghostlore stories cluster around the castle, including one featuring Mary, Queen of Scots herself. Whether anyone has actually seen her wandering the chambers depends on who you ask. The chambers are still there. So are the cannon scars.