This castle from the Middle Ages is the most happening spot in town. We'd visit it for no other reason than to see the panoramic view ★★ of the fjord and the town from its observation deck. But there is so much more here, even cultural events such as classical music concerts presented every Thursday evening (inquire at the tourist office for details). These take place in the Great Hall.

The last royal castle built in Jutland, the domain of King Erik Glipping, the castle became a royal residence, although its original purpose was as a defense against the Duchy of Schleswig to the south. Christian IV spent much of his boyhood here, adding a landmark tower in 1600. Fire and bombardment has destroyed the castle over the years, but it always bounced back, its oldest sections dating from around the mid-1400s. In 1808, Denmark was allied with France under Napoleon Bonaparte. Napoleon commanded Spanish troops because his brother occupied the Spanish throne, and at the time Spanish soldiers were billeted in Koldinghus Slot. The Spanish soldiers, not used to Danish winters, built a roaring fire that not only kept them warm, but set the castle on fire.

Until 1890, the castle was left in ruins. A north wing was restored to house a museum. Reconstruction continued slowly over much of the 20th century, with the Christian IV tower restored by 1935. The exterior has a baroque facade evocative of the 18th century. Today the castle shelters a Historical and Cultural Museum, with exhibitions tracing the town's history. The castle is rich in Romanesque and Gothic sculptures, plus such handcrafted articles as silver, stoneware, and porcelain. A special exhibit documents the wars against Prussia from 1848 to 1850 and in 1864.