The market town sits on the north bank of the River Tees, opposite the smaller settlement of Startforth, twenty-one miles south-west of Durham city. It is named for its castle, and the castle is named for a man: Bernard de Balliol, who in the latter half of the twelfth century rebuilt the earlier earthwork fortifications of his predecessor Guy de Balliol in stone. The land had come to the Balliols after a chain of Norman rebellions in the late eleventh century. After Bishop Walcher of Durham was murdered in 1080 and a further uprising in 1095, William II broke up the Earldom of Northumberland into smaller baronies and granted the Lordship of Gainford to Guy de Balliol. The castle stayed in the Balliol family long enough to be the home of John Balliol, the disputed king of Scotland, before passing through the Nevilles to Richard III via his wife Anne Neville. After Richard fell at Bosworth in 1485 the castle began its slow decline into ruin. English Heritage now manages the remains, which are Grade I listed.

The town's cultural centrepiece is not the medieval castle but the nineteenth-century Bowes Museum, a French chateau planted in extensive grounds at the edge of town. John Bowes - illegitimate son of the 10th Earl of Strathmore and an ancestor of Queen Elizabeth The Queen Mother - married the French actress Josephine Coffin-Chevallier. Together they assembled a collection that would shame many national museums. The building, opened to the public in 1892, holds works by El Greco, Goya, Canaletto, Boucher, and Fragonard. Its most famous object is the eighteenth-century Silver Swan automaton: a life-sized silver bird that periodically preens itself, looks around, and appears to catch a fish from the rippling glass below. It seized up during the 2020 lockdown and underwent a painstaking restoration completed in March 2024, during which a team of horologists spent more than 1,500 hours on the mechanism. John Bowes also ran the Streatlam stud, a remarkably efficient breeding operation of never more than ten mares that produced four Derby winners in twenty years, including West Australian - the first racehorse to win the Triple Crown, in 1853.

Charles Dickens and his illustrator Hablot Browne stayed at the King's Head in the winter of 1837-38, researching Nicholas Nickleby and its grim depiction of the Yorkshire boarding schools. The story goes that Dickens visited William Humphrey's clockmaker's shop opposite the inn and asked who had made a particularly fine clock in the window. The clockmaker said that his boy, Humphrey, had built it. The phrase apparently stuck. Dickens chose Master Humphrey's Clock as the title of his next weekly journal, in which The Old Curiosity Shop and Barnaby Rudge first appeared. Walter Scott visited his friend John Sawrey Morritt at nearby Rokeby Hall and was so taken with Teesdale that he opened his 1813 poem Rokeby with a guard standing watch on the round tower of the Barnard Castle fortress. William Wordsworth, Daniel Defoe, Ralph Waldo Emerson, Hilaire Belloc, J.M.W. Turner, and Bill Bryson all turned up too. The town has been generous with literary visitors for at least four centuries.

Modern Barnard Castle runs on pharmaceuticals. The largest employer is GSK, the multinational pharmaceutical company, which has operated a manufacturing facility on the town's outskirts since the mid-twentieth century. It employs around 1,000 people and has invested £80 million in the plant since 2007. The site sits on what was once Barnard Castle's second railway station, closed under the Beeching cuts in 1964 along with the rest of the South Durham and Lancashire line that ran west to Tebay. The town's older railway closed even earlier. In May 2020 Barnard Castle became briefly the most discussed place name in Britain when Dominic Cummings, Boris Johnson's chief adviser, was found to have driven there from London during the pandemic lockdown - a journey he explained at a Downing Street press conference as a test of his eyesight before driving the family car back to London the next day. The phrase eye test entered the British political lexicon. Barnard Castle, in turn, entered a tourism conversation it had not particularly sought. The Witham Arts Centre still puts on its drama and music; the annual Meet carnival still runs in late May; and the Silver Swan performs at 2pm every day, preening above its glass river once more.