It began as Krogen — 'the hook' — a blunt fortress thrown up in the 1420s by Eric of Pomerania on Ørekrog, a sandy spit reaching out from Zealand toward Scania. Eric introduced the Sound Dues in 1429 and needed something to enforce them. Krogen was a square curtain wall with stone buildings inside it: the king's residence in the northeast corner, a vaulted banqueting hall in the southwest, probably a chapel in the southeast. Paired with Helsingborg Castle opposite and a guard ship between, it closed the Sound. Large sections of Krogen's walls are still inside the building you see today, buried in later masonry.

Artillery improved, and by 1570 the medieval fortress was obsolete. Frederick II first modernised the defences — the Flemish architect Hans Hendrik van Paesschen finished the bastions in 1577 — then rebuilt the castle itself between 1574 and 1585, joining Krogen's separate buildings into three continuous wings. Halfway through, ambition escalated. Anthonis van Obbergen took over as master builder in 1578, the south wing gained a storey to carry an enormous new ballroom above the chapel, and the west and north wings were raised to match. The exterior was faced in Scanian sandstone and roofed in copper, and when the fortification work was done the castle took a new name: Kronborg.

There is no record of William Shakespeare ever leaving England. He almost certainly never saw Kronborg, never crossed the Sound, never stood on the platform where he set the opening scene of Hamlet. What he had was reputation. Kronborg was new, famous and talked about across northern Europe in exactly the years he was writing. He took the town's English name, Elsinore, and gave it back to the world transformed. The castle has been repaying the compliment ever since. Hamlet was first staged here in 1816, on the two-hundredth anniversary of Shakespeare's death, performed by soldiers of the garrison in the telegraph tower at the southwest corner. Professional companies followed, and the roll of visiting princes runs from Laurence Olivier and John Gielgud through Christopher Plummer and Derek Jacobi to David Tennant and Jude Law in 2009. A play about a Denmark that never existed is now performed in the Denmark that does.

On the night of 24-25 September 1629 two workmen were careless and most of the castle burned. Only the chapel came through, saved by the strength of its vaults. Christian IV rebuilt hard and fast — work was under way by 1631 under Hans van Steenwinckel the Younger, and by 1639 the exterior was magnificent again, deliberately unchanged — but the interiors never recovered their first splendour. Worse followed. In 1658 a Swedish army under Carl Gustaf Wrangel besieged and took the castle, and the occupiers stripped it: the courtyard fountain, Frederick II's canopy, many of the great ceiling paintings Christian IV had commissioned. A crownwork and a ring of ramparts went up between 1688 and 1690, after which Kronborg was reckoned the strongest fortress in Europe — a title it earned only after being proved otherwise.

From 1739 the castle held prisoners as well as soldiers. Men sentenced to hard labour on the fortifications were sorted into two categories by the crime that brought them there: those with lighter sentences could work outside the walls, while those convicted of violence, murder or arson served their whole term inside the ramparts. Beyond that, life was identical: chains at all times, and nights in cold, wet cellars beneath what was by then one of the great showpieces of Europe. Between 17 January and 30 April 1772 a very different prisoner was held here: Caroline Mathilde, queen of Denmark and sister of George III, confined after her affair with the royal physician Johann Friedrich Struensee was exposed. From 1785 the castle was purely military, and the army did not leave until the 1920s.

In the casemates, in the dark under the ramparts, a man sits asleep with his head on his hands and his beard grown down into the stone. This is Holger Danske — Ogier the Dane — carried off to Avalon by Morgan le Fay in the Arthurian telling, returned to rescue France, and finally come to rest at Kronborg to wait until Denmark needs him. The statue is by H. P. Pedersen-Dan and dates from 1907, younger than the legend by several centuries and no less potent for it. Above ground, the ballroom completed in 1582 still runs its full 62 metres; it was the largest hall in northern Europe when it was new. The Sound Dues were abolished in 1857 and Kronborg's reason for existing evaporated with them. After the army withdrew the castle was restored, opened to the public in 1938, and inscribed on the UNESCO World Heritage List in 2000.