The ruins of Haderslevhus — also known as Hansborg — sit on high ground near the fjord, a few minutes' walk from the cathedral. What survives from one of medieval Jutland's most important royal castles is modest: some foundation walls and part of a tower, set in a small park. It is not a dramatic ruin on the scale of a Scandinavian stave church, but the historical significance is real and the views over the fjord from the castle hill are the best in town.
The site is freely accessible and interpretive panels (in Danish and German) explain the castle's history. For a quiet stop — an excuse to look at old stones and admire the fjord — it is worth the short walk from the town centre.