The cathedral was built in the second half of the 12C. It stands on the main square, 1.5m below the present ground level. From the top of its square tower there is a remarkable view of the town and the surrounding tidal flats. The south portal is a rare example of Danish Romanesque sculpture. Note, particularly, its carved tympanum depicting the Descent from the Cross.
Inside, there is a wide nave flanked by aisles and surmounted by a dome. You can see medieval and modern frescoes as well as mosaics.

The restored old town, overlooked by the cathedral, stands high up on winding cobbled streets lined with old houses. One of the oldest buildings (late 16C), at no 20 Slotsgade, houses a ceramic collection, Ehlers Samlingen. The Slesvigske Vognsamling in Sejlstensgyde has a collection of horse-drawn carriages from the 17C, 18C and 19C. Finds from local archaeological digs are exhibited at the Haderslev Museum in Dalgade. Opposite, a group of 17C buildings constitute an open-air museum.

The King of Denmark built this castle in the 13C for defensive reasons. In the Middle Ages a fortified town stood here, but nothing remains of it: the oldest parts date from the 15C. When it became a royal residence in the 16C, it lost its fortifications. Frederik IV altered it, but it was then destroyed by a fire in 1808. The originality of its restoration (20C) lies in the alliance of modern structures with the ancient ruins, as seen in the great hall, the chapel and the south wing. A museum of Danish crafts and decorative art is now housed here.