The Schlei is the defining geographical feature of Schleswig — a narrow arm of the Baltic Sea that reaches 42 kilometres inland, tapering as it goes until it becomes almost river-like near the town. Schleswig sits at the head of the Schlei, where the water is at its narrowest and most sheltered. The fjord is not just scenic backdrop; it is actively used for sailing, kayaking, fishing, and swimming in summer.
Kayak rental is available in town, and paddling out along the Schlei is one of the best ways to understand why this location was so strategically valuable to both the Vikings at Haithabu and to successive rulers who built their palaces here. Boat trips are also possible from the town quay. On the southern shore, the earthworks of the Danevirke — the great Viking-era defensive wall — run roughly parallel to the Schlei for several kilometres.