Koldinghus exists because of a family quarrel that hardened into two centuries of war. In 1252 King Abel of Denmark was killed on an expedition into Friesland. The obvious successor, Valdemar III, Duke of Schleswig, was sitting in a prison cell in Cologne, so the crown went instead to Abel's brother Christoffer I — who was murdered in 1259, leaving his ten-year-old son Erik V to inherit a throne and a feud together. Abel's sons, led by Erik I, Duke of Schleswig, wanted both back.

Out of that came the decision to fortify the kingdom's southern border. A hill in the middle of the town of Kolding was chosen, a moat dug, wooden palisades raised. The first Koldinghus stood finished in 1268. It was a border castle built to face a neighbour who happened to be a cousin, and the rebels came anyway: the castle was besieged in 1341 and again in 1368–69 by Holstein and Jutish forces.

Each century added a wing. The oldest surviving stonework is the north range facing the castle lake, raised by Christoffer III (1441–1448). Christian I (1448–1481) built the west. Christian III (1503–1559) added the south side and the small courtyard towers, converting a fortress into a residence at exactly the moment when cannon had made thick walls less valuable than warm rooms. His son Frederick grew up here, and Christian III himself died here on 1 January 1559.

Christian IV, king from 1588, crowned the west wing with the Giant Tower, named for the four heroes of antiquity standing on its parapet: Hannibal, Hector, Scipio and Hercules. Only Hercules is still up there. Hannibal and Hector were crushed in the fire of 1808, and Scipio came down in a storm in 1854.

War reached the castle too. On 25 December 1658, Polish troops under Stefan Czarniecki came ashore from a naval landing, stormed Koldinghus and drove the Swedish garrison out of Kolding.

By 1808 Denmark had tied itself to Napoleon, and the alliance brought 30,000 French and Spanish soldiers north for a planned campaign to recover the Scanian provinces lost to Sweden a century and a half earlier. Spanish troops were quartered at Koldinghus that winter under the French marshal Jean-Baptiste Bernadotte — who would, in one of history's stranger turns, later become king of Sweden and Norway.

Jutland in March is not Andalusia. By every account the soldiers kept the stoves and furnaces roaring, and reportedly fed furniture into them when the wood ran short. Add far more people than the castle was built to hold, and the outcome starts to look less like bad luck than arithmetic. Fire broke out in the small hours of the night between 29 and 30 March 1808. By the time the alarm went up, the main buildings could not be saved.

Denmark lost the Napoleonic Wars, and the money for rebuilding went with them. So Koldinghus stayed a ruin — and, as ruins do, became an attraction in its own right. Hans Christian Andersen came to look at it. A museum was established on the site in 1890. In the 1930s the great tower was rebuilt in brick for the plain reason that it was about to fall over.

The real decision came much later. From 1976 the architects Inger and Johannes Exner worked to a principle that was genuinely radical for its moment: reversibility. Nothing they added would harm what survived, and everything that survived — including the damage — was treated as equally historic. Rather than reconstruct lost interiors from guesswork, they slid new structures into the ruin: wooden columns and new staircases that carry real weight while announcing themselves unmistakably as modern. The work ran under museum director Poul Dedenroth-Schou, finished in 1993, and won a Europa Nostra Award that same year.

Koldinghus today holds Danish furniture from the sixteenth century to the present, Romanesque and Gothic church art, older Danish painting, and collections built around ceramics and silver, alongside changing thematic exhibitions. In December 2009, timed to the United Nations climate conference, it hosted e-Collection with Kolding School of Design and the Trapholt art museum — innovative motorcycles, jewellery and eco-minded design in a castle that had spent most of two centuries as an empty shell. Crown Prince Frederik opened it.

The exhibitions rotate. The argument the building makes does not: a ruin is not a mistake waiting to be corrected.